Creating GUI won't work

ItemSign:

public boolean onItemUse(ItemStack par1ItemStack, EntityPlayer par2EntityPlayer, World par3World, int par4, int par5, int par6, int par7, float par8, float par9, float par10)
    {
        //Rotate the sign correctly

                par3World.setBlock(par4, par5, par6, Blocks.Sign.blockID, par7, 3);
                
                if (!par2EntityPlayer.capabilities.isCreativeMode)
                --par1ItemStack.stackSize;
                
//Don't Quite know, what's correct.
//              FMLNetworkHandler.openGui(par2EntityPlayer, Main.instance, 0, par3World, par4, par5, par6); 
                if (!par3World.isRemote)
                par2EntityPlayer.openGui(Main.instance, 0, par3World, par4, par5, par6);

                return true;
        }

BlockSign (Copied from a Tutorial)

package mod.classes.advanced;

import net.minecraft.block.BlockContainer;
import net.minecraft.block.material.Material;
import net.minecraft.client.renderer.texture.IconRegister;
import net.minecraft.entity.EntityLiving;
import net.minecraft.item.ItemStack;
import net.minecraft.tileentity.TileEntity;
import net.minecraft.util.AxisAlignedBB;
import net.minecraft.util.MathHelper;
import net.minecraft.world.IBlockAccess;
import net.minecraft.world.World;
import cpw.mods.fml.relauncher.Side;
import cpw.mods.fml.relauncher.SideOnly;

public class ModBlockSign extends BlockContainer {

public ModBlockSign(int par1, Material par2Material) {
	super(par1, par2Material);
	this.setBlockBounds(0.0F, 0.0F, 0.0F, 1.0F, 1.0F, 1.0F);
}

    @Override
    public TileEntity createNewTileEntity(World world) {
            return new TileEntitySign();
    }

    @Override
    public int getRenderType() {
            return -1;
    }

    @Override
    public boolean isOpaqueCube() {
            return false;
    }

    public boolean renderAsNormalBlock() {
            return false;
    }

    //I think you don't need this:
    public void registerIcons(IconRegister icon) {
            this.blockIcon = icon.registerIcon("moresigns:sign");
    }
    
    public void onBlockPlacedBy(World par1World, int par2, int par3, int par4, EntityLiving par5EntityLiving, ItemStack par6ItemStack)
    {
            int l = MathHelper.floor_double((double)(par5EntityLiving.rotationYaw * 4.0F / 360.0F) + 2.5D) & 3;
            par1World.setBlockMetadataWithNotify(par2, par3, par4, l, 2);
    }
    
    @SideOnly(Side.CLIENT)
    public AxisAlignedBB getSelectedBoundingBoxFromPool(World par1World, int par2, int par3, int par4)
    {
        this.setBlockBoundsBasedOnState(par1World, par2, par3, par4);
        return super.getSelectedBoundingBoxFromPool(par1World, par2, par3, par4);
    }

    public void setBlockBoundsBasedOnState(IBlockAccess par1IBlockAccess, int par2, int par3, int par4)
    {
        int l = par1IBlockAccess.getBlockMetadata(par2, par3, par4);
        float f = 0.0F;
        float f1 = 1.0F;
        float f2 = 0.0F;
        float f3 = 1.0F;
        float f4 = 0.125F;
        this.setBlockBounds(0.0F, 0.0F, 0.0F, 1.0F, 1.0F, 1.0F);
        if (l == 2)
        {
        	this.setBlockBounds(f2, f, 1.0F - f4, f3, f1, 1.0F);
        }

        if (l == 3)
        {
        	this.setBlockBounds(f2, f, 0.0F, f3, f1, f4);
        }

        if (l == 4)
        {
        	this.setBlockBounds(1.0F - f4, f, f2, 1.0F, f1, f3);
        }
        
        if (l == 5)
        {
        	this.setBlockBounds(0.0F, f, f2, f4, f1, f3);
        }
    }

    public AxisAlignedBB getCollisionBoundingBoxFromPool(World par1World, int par2, int par3, int par4)
    {
        return null;
    }
}

GuiHandler:

package mod.classes.gui;

import mod.Main;
import mod.classes.advanced.TileEntitySign;
import net.minecraft.client.gui.inventory.GuiEditSign;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.tileentity.TileEntity;
import net.minecraft.world.World;
import cpw.mods.fml.common.network.IGuiHandler;
import cpw.mods.fml.common.network.NetworkRegistry;

public class ModGuiHandler implements IGuiHandler {

@Override
public Object getServerGuiElement(int ID, EntityPlayer player, World world, int x, int y, int z) {
	TileEntity entity = world.getBlockTileEntity(x, y, z);
	System.out.println("Server");

	switch(ID)
	{	
	case 0:	if (entity instanceof TileEntitySign)
				return null; //Don't know what to return here; think that's the mistake
			else
				return null;

	default: return null;
	}	
}

@Override
public Object getClientGuiElement(int ID, EntityPlayer player, World world, int x, int y, int z) {
	TileEntity entity = world.getBlockTileEntity(x, y, z);
	System.out.println("Client");

	switch(ID)
	{	
	case 0:	if (entity instanceof TileEntitySign)
				return new GuiEditSign((net.minecraft.tileentity.TileEntitySign) world.getBlockTileEntity(x, y, z));
			else
				return null;

	default: return null;
	}		
}

public ModGuiHandler() {
	NetworkRegistry.instance().registerGuiHandler(Main.instance, this);
}

}

GUI: (Copied from Vanilla Sign)

package mod.classes.gui;

import cpw.mods.fml.relauncher.Side;
import cpw.mods.fml.relauncher.SideOnly;
import net.minecraft.block.Block;
import net.minecraft.client.gui.GuiButton;
import net.minecraft.client.gui.GuiScreen;
import net.minecraft.client.multiplayer.NetClientHandler;
import net.minecraft.client.renderer.tileentity.TileEntityRenderer;
import net.minecraft.network.packet.Packet130UpdateSign;
import net.minecraft.tileentity.TileEntitySign;
import net.minecraft.util.ChatAllowedCharacters;
import org.lwjgl.input.Keyboard;
import org.lwjgl.opengl.GL11;

@SideOnly(Side.CLIENT)
public class EditSignGui extends GuiScreen
{
    /**
     * This String is just a local copy of the characters allowed in text rendering of minecraft.
     */
    private static final String allowedCharacters = ChatAllowedCharacters.allowedCharacters;

    /** The title string that is displayed in the top-center of the screen. */
    protected String screenTitle = "Sign Message (Redstone off):";

    /** Reference to the sign object. */
    private TileEntitySign entitySign;

    /** Counts the number of screen updates. */
    private int updateCounter;

    /** The number of the line that is being edited. */
    private int editLine;
    
    /** Editing the Redstone-active Text or not */
    private boolean redstoneText = false;

    /** "Done" button for the GUI. */
    private GuiButton doneBtn;
    private GuiButton redstonePowered;

    public EditSignGui(TileEntitySign par1TileEntitySign)
    {
        this.entitySign = par1TileEntitySign;
    }

    /**
     * Adds the buttons (and other controls) to the screen in question.
     */
    public void initGui()
    {
        this.buttonList.clear();
        Keyboard.enableRepeatEvents(true);
        this.buttonList.add(this.doneBtn = new GuiButton(0, this.width / 2 - 100, this.height / 4 + 120, "Done"));
        this.buttonList.add(this.redstonePowered = new GuiButton(1, this.width / 2 - 100, this.height / 4 + 80, "Redstone state on/off"));
        this.entitySign.setEditable(false);
    }

    /**
     * Called when the screen is unloaded. Used to disable keyboard repeat events
     */
    public void onGuiClosed()
    {
    	//TODO: Need to be edited! 
    	/*
        Keyboard.enableRepeatEvents(false);
        NetClientHandler netclienthandler = this.mc.getNetHandler();

        if (netclienthandler != null)
        {
            netclienthandler.addToSendQueue(new Packet130UpdateSign(this.entitySign.xCoord, this.entitySign.yCoord, this.entitySign.zCoord, this.entitySign.signText));
        }

        this.entitySign.setEditable(true); */
    }

    /**
     * Called from the main game loop to update the screen.
     */
    public void updateScreen()
    {
        ++this.updateCounter;
    }

    /**
     * Fired when a control is clicked. This is the equivalent of ActionListener.actionPerformed(ActionEvent e).
     */
    protected void actionPerformed(GuiButton par1GuiButton)
    {
        if (par1GuiButton.enabled)
        {
            if (par1GuiButton.id == 0)
            {
                this.entitySign.onInventoryChanged();
                this.mc.displayGuiScreen((GuiScreen)null);
            }
            
            if (par1GuiButton.id == 1)
            {
            	
            }
        }
    }

    /**
     * Fired when a key is typed. This is the equivalent of KeyListener.keyTyped(KeyEvent e).
     */
    protected void keyTyped(char par1, int par2)
    {
        if (par2 == 200)
        {
            this.editLine = this.editLine - 1 & 3;
        }

        if (par2 == 208 || par2 == 28 || par2 == 156)
        {
            this.editLine = this.editLine + 1 & 3;
        }

        if (par2 == 14 && this.entitySign.signText[this.editLine].length() > 0)
        {
            this.entitySign.signText[this.editLine] = this.entitySign.signText[this.editLine].substring(0, this.entitySign.signText[this.editLine].length() - 1);
        }

        if (allowedCharacters.indexOf(par1) >= 0 && this.entitySign.signText[this.editLine].length() < 15)
        {
            this.entitySign.signText[this.editLine] = this.entitySign.signText[this.editLine] + par1;
        }

        if (par2 == 1)
        {
            this.actionPerformed(this.doneBtn);
        }
    }

    /**
     * Draws the screen and all the components in it.
     */
    public void drawScreen(int par1, int par2, float par3)
    {
        this.drawDefaultBackground();
        this.drawCenteredString(this.fontRenderer, this.screenTitle, this.width / 2, 40, 16777215);
        GL11.glPushMatrix();
        GL11.glTranslatef((float)(this.width / 2), 0.0F, 50.0F);
        float f1 = 93.75F;
        GL11.glScalef(-f1, -f1, -f1);
        GL11.glRotatef(180.0F, 0.0F, 1.0F, 0.0F);
        Block block = this.entitySign.getBlockType();

        if (block == Block.signPost)
        {
            float f2 = (float)(this.entitySign.getBlockMetadata() * 360) / 16.0F;
            GL11.glRotatef(f2, 0.0F, 1.0F, 0.0F);
            GL11.glTranslatef(0.0F, -1.0625F, 0.0F);
        }
        else
        {
            int k = this.entitySign.getBlockMetadata();
            float f3 = 0.0F;

            if (k == 2)
            {
                f3 = 180.0F;
            }

            if (k == 4)
            {
                f3 = 90.0F;
            }

            if (k == 5)
            {
                f3 = -90.0F;
            }

            GL11.glRotatef(f3, 0.0F, 1.0F, 0.0F);
            GL11.glTranslatef(0.0F, -1.0625F, 0.0F);
        }

        if (this.updateCounter / 6 % 2 == 0)
        {
            this.entitySign.lineBeingEdited = this.editLine;
        }

        TileEntityRenderer.instance.renderTileEntityAt(this.entitySign, -0.5D, -0.75D, -0.5D, 0.0F);
        this.entitySign.lineBeingEdited = -1;
        GL11.glPopMatrix();
        super.drawScreen(par1, par2, par3);
    }
}

package mod;

import mod.classes.advanced.TileEntitySign;
import mod.initialization.Blocks;
import mod.initialization.Configurations;
import mod.initialization.Items;
import mod.initialization.Languages;
import mod.initialization.Registrations;
import mod.proxies.ClientProxy;
import mod.proxies.CommonProxy;
import cpw.mods.fml.common.Mod;
import cpw.mods.fml.common.Mod.*;
import cpw.mods.fml.common.SidedProxy;
import cpw.mods.fml.common.event.FMLInitializationEvent;
import cpw.mods.fml.common.event.FMLPostInitializationEvent;
import cpw.mods.fml.common.event.FMLPreInitializationEvent;
import cpw.mods.fml.common.network.NetworkMod;
import cpw.mods.fml.common.registry.GameRegistry;
import cpw.mods.fml.relauncher.Side;


@Mod(modid = Main.modID, name = Main.modName, version = "1.0.0")
@NetworkMod(clientSideRequired = true, serverSideRequired = false)

public class Main {

public static final String modID = "moreSigns";
public static final String modName = "More Signs Mod"; 

public static int ItemStartID;
public static int BlockStartID;

@Instance(modID)
public static Main instance;

@SidedProxy(clientSide = "mod.proxies.ClientProxy", serverSide = "mod.proxies.CommonProxy")
public static CommonProxy proxy;

@EventHandler
public void PreInit(FMLPreInitializationEvent e){
	Configurations.load(e);
}

@EventHandler
public void Init(FMLInitializationEvent e){
	Items.init(ItemStartID); //unimportant
	Blocks.init(BlockStartID); //unimportant
	Languages.load(); //unimportant
	Registrations.init(); 
	proxy.renderThings();
}

@EventHandler
public void PostInit(FMLPostInitializationEvent e){

}
}

public static void init()
{
	GameRegistry.registerTileEntity(TileEntitySign.class, "tileEntitySign");
	new ModGuiHandler();
}

public class ClientProxy extends CommonProxy{

public void renderThings(){
	 ClientRegistry.bindTileEntitySpecialRenderer(TileEntitySign.class, new TileEntitySignRenderer());
}

}

Write, if you need more code!