"Prior to the release of the Xbox 360 version of the Exterminatus DLC it came to our attention that there was a connectivity issue that needed to be fixed. We have taken the difficult decision to delay the title update and DLC release to repair this issue
and ensure that the mode is functioning as intended when we release it. We will share a projected live date for this content on the Space Marine community site as soon as possible."
Posted by Auburok over on x360a.org
Looks like it's going to be a while before the Space Marine DLC will be available.