I spoke with a Xbox LIVE agent and he suggested contacting EA support to get the registration code. So I'm pretty much out of luck here.
@oDovahkiin If you purchased it thru Microsoft they are the ones that would issue the code. Just like if you bought the game thru Amazon. You get a code to redeem the game in origin. You could try EA customer service but if you never registered the game I have no idea how they can track the sale but time is all you have to loose. Maybe they know how the Microsoft Corp. handles those sales. If Microsoft just passes the sale to EA why have it in Games for Windows (unless it is the xbox version) - I'm not sure. I personally wouldn't buy a game like Sims 3 (expansion and stuff packs) as even Steam has had problems. This game is complicated enough without introducing more issues. Good luck
