I feel like a total noob asking this question, but I downloaded the expansion and haven't seen any in-game references (added quests, etc) for how to actually begin the expansion. Any (like very obvious) help would be much appreciated.
You can go to the new area by talking to one of the captains in the harbor on Windhelm (This allows you to do any side quests that are not part of Dragonborn's main quest) but to activate the main quest you need to be attacked by Cultists at any one of the major cities in Skyrim accusing you to be a fake "Dragonborn".
You won't be attacked by any however until you've at least finished the "Meet the Greybeards" portion of Skyrim's main quest and have been given the mission to retrieve the Horn of Jur-something (I forgot the name but it's after they teach you the Whirlwind Shout)
For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill. ~Sun Tzu