I suggest you check with the mod author on steam/nexus.
I HAVE run into instances of mods not taking effect on loading saved games. It seems completely random. Sometimes a save game will work, sometimes it will not. So you might try playing a bit, saving, and then enabling the add-on mod.
It's also entirely possible that the mod author did not account for attempting to use the mod in an existing game, so you should check with him on that.