I'm not sure why you guys are having this problem. I did exactly what that guy said to do and it came up right off the marketplace. I live in Wisconsin, so I'm not sure why you guys should have this problem.
Dawnguard wasn't showing up in the extras bit but how I found it was starting the game and on the main menu going to add ons and opening the marketplace from there.
for all you guys who dont have it yet, when the dashboard updates go out it takes a second to hit every region in the world. hence why some have it and some dont. youll have it very soon. patience