In realms, you are unable to put in addons in any way.
If you try to add in a world with an addon: The world will remain in uploading world indefinitely, even if it says the world transfer is complete.
If you try to add an addon to an existing realm: It will add the addon into the realm, but the addon will have no effect on the game. No items will show up from the addon, no mobs will be changed, etc etc...