How can i get my expansion packs back for new Xbox 360 slim console?

bought the new console and wanna play halo 3 but all my expansion level packs that i have gone, anyway to retrieve them? thanks

Step 1: Make sure you have Xbox Live Gold membership set up with the account that you originally purchased add-on content for.
Step 2: Open up the Xbox Guide and go to the Marketplace blade.
Step 3: Select “Game Marketplace” and hit A.
Step 4: Navigate to Game Add-ons in the Game Marketplace.
Step 5: Select Browse all Game Add-ons, then navigate to the game you’re looking to download your content from. (Ex. Halo 3 Mythic Map pack: Browse–>H–>Halo 3–>All Downloads–>Mythic Map Pack 1)