Where can you download the Halo 2 Map Packs for XBOX?

If you buy the card from a retailer, such as target, they'll put a receipt on the back that gives you the code. Then come home fire up your Xbox 360, hit the middle button, go all the way to the left and click redeem code and put the 25 digit code in. If you're buying from the marketplace just buy it and vwallah you have the 3 maps.