Did you try to enter it at the XLive "Redeem Code" option? The instructions in my email have it listed as an in-game code that you enter there (Main Menu, Downloadable Content, Enter Coupon Code, then enter it and select Submit).
I have those instructions too, I didn't think of it that way (in-game vs. Xbox Marketplace). I was just trying to get the map downloaded now but I guess that won't happen without the game! Thanks for the clarification.