Custom items in Iris
Adding items from MMOItems is simple. In your loot tables (or other places where you can put items, such as as equipment of mobs), if you want to add an MMOItems item which has type "sword" and name "excalibur", write mmoitems_sword:excalibur in the place where you would otherwise write minecraft:diamond_sword.
If you are using the Iris VSCode Workspace the autocompletions (ctrl + space) will include the items as well.