My girlfriend and I share an Xbox. She has a gold XBL account. I have a free/silver account. She bought and downloaded all DLC for FO3. It all operates fine when she is logged on. When I log on, none of them are accessible (I have spent hours ingame, outside the vault, plenty of time for the quests to pop up). I don't get the quest updates, none of the ingame locations are added. The DLC achievements are added to the list of possible achievements for FO3 and the "loading downloadable content" notification pops up when the game is first loaded. I'm able to play all other content that my GF has purchased, from XBLA games, to DLC for other titles.

How do I get the DLC for fallout to work?

I think that what she bought is tied to her account only.Edit: Or not, if you can play DLC from other games. Might want to contact Microsoft.