Instructions
Insert the game disc for "Call of Duty: World at War" into your game console's disc tray and close the tray. Select the game to initialize it if it doesn't start automatically.
Click the "Yes, Update Now" option when prompted to update the game. The game update that removes the prerequisites for the zombie mode will now be downloaded and installed.
Wait for the main menu of "Call of Duty: World at War" to load after the update finishes installing and then select "Nazi Zombies."
Select your map, choose your game options (one player or two player split-screen) and then start the game. Zombie mode will now start.