You Must Complete A Quest Before You Start Far Harbor
Before you fire up Fallout 4 and Far Harbor there are some things you should know.
Getting a Clue is one of the intial quests in the main quest line so most of you have probably completed it in your game. If you haven't or if you need more information about it, check out the Getting a Clue wiki. It'll help get you ready for Far Harbor.
Once you've got that done you'll want to listen to Valentine's Detective Agency Radio and the quest "Far From Home" will begin.
A few other notes:
- The developer also notes that "you can also access the DLC content by locating the Nakano residence at the northeast corner of the Commonwealth." This area was previously inaccessible.