As soon as you purchased and downloaded the Far Harbor DLC for Fallout 4, you will get a quest immediately titled “Far From Home”. This quest is the main questline for the Far Harbor DLC.
Your first objective in this Far From Home quest is to listen to Valentine’s Detective Agency Radio signal. Go to the radio and simply listen to Ellie asking you to come visit the agency in Diamond City.
By talking to Ellie, you will learn that a Mr. Nakano is asking the agency to help find her daughter. You will be then asked to go in the northeastern part of the map.
If you have been to the Witchcraft’s Museum, simply fast travel there and head north. If not, look at the map. Please note that the Nakano’s residence is located farther north of the map. Don’t be surprised if you’ve gone past the world map.