Add an enchanted book to a loot table

You can't do this with the MCreator software. However, it is quite easy to do by making your loot table, then changing the code (the ID would be enchanted_book{StoredEnchantments:[{id:*name_of_mod*:*name_of_enchantment*,lvl:1}]} (delete the *'s in there as well).

For example, with a custom curse from my mod, I would do enchanted_book{StoredEnchantments:[{id:brine:drowning_curse,lvl:1}]}

i tried doing this but now the chests just have no items in them

Is the Boss from your Mod? If not then you need to use this Plugin: Loot modifier.

This allows you to add your Loot Table into existing ones without replacing it. 

Also which MC Version are you using?

But you use this Loot Table Generator for your Situation. There you need to add "Functions" to your Item and just copy the Functions Code.