Wow... Big avatar.
First, you need a credit/debit/visa gift card attatched tou your account. To do this go to, the Marketplace blade> Account management> Memberships> Click the one you want to modify> Change Payment Option> Add crdit card> Follow on-screen prompts until complete. Now click on it in the payment options list. Now go into Halo 2, choose Download new content, then select the blastacular map pack and let it download. It's $4.