Take note: If you have played a recent EA game with an online component (like Battlefield 1943) on your Xbox 360, chances are you already have an EA.com account linked to your Gamertag. If this is the case, do not create a new account on BioWare's or EA's site to redeem your code.
Xbox 360 owners that have played an EA game online and have their Gamertags tied to an account should not input their codes until they can log into their EA.com accounts. If you've already tied your Mass Effect 2 DLC code to a new account, the only way we know to get the game content is to transfer your new account over to an existing one, which requires getting in contact with EA.
We're currently contacting the publisher to see if EA can implement a streamlined process to transfer accounts and will update as soon as we hear back.
Update: We've placed EA's suggestions on fixing various Mass Effect 2 DLC issues after the break.