The DLC content for Dragon Age: Origins named Return to Ostagar was subsequently pulled from XBOX Live earlier this month due to having too many bugs. It seems BioWare have fixed the faulty update and the DLC is now back and available to download via the XBOX Live marketplace. If you’ve already downloaded Return to Ostagar you don’t have to download it again. All that you need to do is install the new update and your problems will be solved.
BioWare have also said its still testing the DLC content for Dragon Age: Origins for the PC and PlayStation 3 and the DLC will hopefully be available for those formats later this year.