Ok, I am making a "syringe" which holds the "DNA" of minecraft mobs using their UUID's, but when I try to get the entity based on its UUID, I don't get any entity with the same UUID, even if their chunks are loaded and the entities are not dead. Here's my code

 

package me.Aregak2005.mc.lotostuff.tools.wtrap;

import cpw.mods.fml.common.registry.GameRegistry;
import me.Aregak2005.mc.lotostuff.Constants;
import net.minecraft.client.renderer.texture.IIconRegister;
import net.minecraft.entity.Entity;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.util.EnumChatFormatting;
import net.minecraft.util.IIcon;
import net.minecraft.util.StatCollector;

import java.util.List;

/**
* Created by DeLL on 30/07/2016.
*/
public class Syringe extends Item {

    public IIcon empty,full;

    @Override
    public IIcon getIconIndex(ItemStack stack) {
        return stack.hasTagCompound()?full:empty;
    }
    @Override
    public IIcon getIcon(ItemStack stack, int pass) {
        return stack.hasTagCompound()?full:empty;
    }

    @Override
    public void registerIcons(IIconRegister reg) {
        empty = reg.registerIcon("ltstf:syringe_empty");
        full = reg.registerIcon("ltstf:syringe_full");
    }

    public Syringe() {
        setUnlocalizedName("syringe");
        setCreativeTab(Constants.tabMetal);
        setTextureName("ltstf:syringe_empty");
        setMaxStackSize(1);
        GameRegistry.registerItem(this,this.getUnlocalizedName().substring(5));
    }

    @Override
    public void addInformation(ItemStack s, EntityPlayer p, List l, boolean b) {
        if(s.hasTagCompound()){
            Entity e = null;
            for(Entity en : (List<Entity>)p.worldObj.getLoadedEntityList()){
                if(en.getUniqueID().toString().equals(s.getTagCompound().getString("uniqueID"))){
                    e = en;
                }
            }

            l.add(s.getTagCompound().getString("uniqueID"));
            if(e==null) l.add(EnumChatFormatting.DARK_RED +  StatCollector.translateToLocal("lang.entitynotfound.txt"));
            else{
                l.add(String.format("X: %d", e.posX));
                l.add(String.format("Y: %d", e.posY));
                l.add(String.format("Z: %d", e.posZ));
            }
        }
    }

    @Override
    public boolean onLeftClickEntity(ItemStack stack, EntityPlayer player, Entity entity) {
        if(player.worldObj.isRemote) return false;
        if(!stack.hasTagCompound()) {
            if (entity.myEntitySize == Entity.EnumEntitySize.SIZE_2) {
                NBTTagCompound t = new NBTTagCompound();
                t.setString("uniqueID",entity.getUniqueID().toString());
                stack.setTagCompound(t);
                assert t.getString("uniqueID").equals(entity.getUniqueID().toString());
                stack.setStackDisplayName(StatCollector.translateToLocal("item.syringe.full"));
            }
        }
        return false;
    }
}

Is it adding the Error message to the itemstack?

Yeah I would like to upgrade but i can't (I hate the new renering system where you have to make a json file for every item/block. its annoying). Is there a way to get the UUID on the server? or sync them manually?

Well if you wanted to sync manually you could try to use packets, but eventually you are going to have to update. Are you planning on just addingInfo on hover to the ItemStack or was that just to test.