Spoiler below on how the DLC triggers.

The quest triggers with a pair of "cultists' showing up and trying to kill you for claiming to be dragonborn. Then you read a letter and it sends you to windhelm where you take a boat to the new island.

So if you mod the enemies you may have a mod that prevents them from appearing in the game. You will probably have to wait for whoever did the mod to update it for the DLC.