Instructions
Log into your Xbox Live account on your Xbox 360.
Open the Games Marketplace, then scroll to "Add-ons and Extras."
Scroll up to "Titles A-Z" and select "H."
Highlight "Halo 3" and press the "A" button to confirm.
Scroll to the right until you hit the "Extras" category.
Press down on the control stick until you find "Halo 3 Mythic Map Pack."
Press the A button to open the purchase window, then hit "Confirm Download" to purchase. The "Mythic Map Pack" costs 400 Microsoft Points, so you may be prompted to reload your account if you do not have enough Microsoft Points available.
Wait for the map pack to download. Once it has downloaded, you can access the "Sandbox" map any time you play "Halo 3."