I downloaded Dead Money off Xbox Live, and have two things happen:

-Sometimes when I go to the location Northern Passage, it is blocked by boulders, and then sometimes it isn't. It goes away when I fast travel and come back.
-If it isn't blocked with boulders, I enter it, go to the end of the tunnel on Dead Money side, and the text "Exit to Mojave Wasteland" appears, but I can't go through.

I cannot play the DLC.

What are you talking about? What's the Northern Passage got to do with Dead Money? Dead Money is accessed via the Abandoned Brotherhood of Steel Bunker.