Instructions
Turn on your console and ensure it is connected to the Internet. Scroll to the "PlayStation Network" tab on the console's menu, then scroll down to "PlayStation Store" and press "X." Enter your PlayStation Network login details into the relevant text fields if the login screen appears.
Add funds to your PlayStation Store wallet with the "Add Funds" button at the top of the store interface if necessary. Follow the instructions on screen to add funds to your wallet either by credit card or PlayStation Network card.
Highlight "Add-Ons" on the store menu and press "X." Scroll right to the "Disc-Based Game Add-Ons" option and press "X" again. Choose "A -- L Add-Ons" from the next menu and press "X" to select it.
Scroll through the list to "Call of Duty: World at War" and select it with the "X" button. Highlight the map pack or map pack bundle you want to buy and press "X."
Press the "Buy Now" button, followed by "Confirm Purchase" to buy the map pack and start the download process.