Instructions
Turn on your Xbox 360 and load the Xbox Live Marketplace from your console's main menu. This screen will show you content that you can download to your console from the Internet, including updates for games (like the zombie map pack for "Call of Duty").
Select "Games" and use the Xbox 360's virtual keyboard to search for "Call of Duty: World at War" downloadable content. When this list loads, the zombie map pack will appear among the list of add-ons returned. Select its name.
Select "Purchase." This will charge your Xbox Live account for the price of the downloadable content. Select "Install" to install the zombie map pack onto your console. You can now play the zombie-themed maps whenever you insert "Call of Duty: World at War" into your Xbox 360.