Things You'll Need
Instructions
Log into your Xbox LIVE account on your console.
Go to "Game Marketplace" on the console's dashboard menu.
Choose "Game Library." This option lets you browse the games list in order to find updates for your specific game.
Go to "Browse."
Choose "H." A list of games starting with the letter "H" appears.
Choose "Halo 3."
Select "Mythic Map Pack" from the downloads list.
Select "Download Now."