Instructions
Go to the "PlayStation Network" icon on the PS3 main menu. Select "PlayStation Store." Sign into your PSN account. If you don't have an account, select "New Account" and enter the requested information, such as the account name and password, using the on-screen keyboard.
Select the "Search" button. Type "World at War" using the on-screen keyboard and select "Search." Select any of the "Call of Duty: World at War" Map Packs.
Select "Buy Now." If you already have money in your PlayStation Store account, select the "Download Now" button to begin the download. If you don't have money in the account, you will be prompted to add funds. Enter the requested credit card/PlayStation Network card information using the on-screen keyboard. After adding funds to the account, select the "Download Now" button. The content from the map pack will be accessible in-game after the download has finished.