Ok, I got some progress:

 

public class ItemModArmor extends ItemArmor {

public ItemModArmor(String unlocalizedName, ArmorMaterial material, int renderIndex, int armorType) {
	super(material, renderIndex, armorType);

	this.setUnlocalizedName(unlocalizedName);
	this.setCreativeTab(KaktusMod.kaktusmodcreativetab);
}

@Override
public void onArmorTick(World world, EntityPlayer player, ItemStack itemStack) {
	if(itemStack.getItem() == KaktusItems.itemModArmorHelmet)
		effectPlayer(player, Potion.waterBreathing, 0);
}

private void effectPlayer(EntityPlayer player, Potion potion, int amplifier) {
	if(player.getActivePotionEffect(potion) == null || player.getActivePotionEffect(potion).getDuration() <= 1)
		player.addPotionEffect(new PotionEffect(potion.id, 159, amplifier, true, true));
}

@Override
public String getArmorTexture(ItemStack stack, Entity entity, int slot, String type) {
	if(stack.getItem() == KaktusItems.itemModArmorHelmet)
		return Config.MODID + ":textures/models/armor/turtleArmorMaterial_layer_2.png";
	else {
		return Config.MODID + ":textures/models/armor/turtleArmorMaterial_layer_1.png";
	}
}
}

 

I just wrote this code for testing purpose, so I wasn't able to fix the complete problem yet, but I got with that code 2 of 4 items shown in the CreativeTabs.

For more details, I just uploaded the Code to your repository: https://github.com/DasKaktus/KaktusMod

 

Here is a picture of it:

 

I hope this helps a bit.

Bektor

Thank you, we found the main problem, in getSubItems.