I had this problem. For some reason I had to go to the xbox live website, click on "create new account", then try and sign in again. This prompted me to accept a new user agreement, which fixed the problem. I hope this works for you too.
Thugs-4-Less. If it ain't broke, we'll break it. "Marking topics is for virgins." - FPrefect
You have to go on the xboxlive website, sign into your gametag or account thingy and select buy the base game from the marketplace even though its free. Thats all you have to do. Dont need to download the game again if you already have through steam.
Playing: Killing Floor, BFBC2, Team Fortress 2, L4d2 --Yoichi Wada needs to be fired--