To begin Dragonborn, you must travel to Solstheim. This can be done at any points after installing the game add-on content by travelling to the Windhelm docks and locating Gjalund Salt-Sage, a sea captain that may be willing to take you to the island for a small fee.
The “main” questline of Dragonborn will start soon after you have started the main game’s quest “The Horn of Jurgen Windcaller”, after consulting with the Greybeards for the first time. After having started this quest, you will soon run into a group of Cultists when traveling around the world that will discuss Dragonborn’s questline.
My DLC isn’t loading!
If you are playing Dragonborn on an Xbox 360 or Hard Drive that is different from the one the content was originally purchase on, you will need to log into the LIVE-enabled Profile that the content was originally purchased because of with. If this is the case, you will need to do a http://support.xbox.com/en-US/xbox-live/marketplace-and-purchasing/download-content in order to play the content offline.
Additionally, make sure that Skyrim is updated with title update 1.8 or higher. When logged into Xbox LIVE, the game will automatically prompt you to download this update if you don’t already have it.
Why can’t I get the content in (X) language?
The language of the content is determined by your Xbox LIVE region.