Instructions
Load the latest update for the PS3 console. Power on the PS3, and push the "Right" directional button until the "Network" option loads. Select "PlayStation Store," and press the "X" button. If an Internet upgrade is available, you will be prompted to download it. Accept the available download, and wait for the information to download to the gaming system.
Download the latest game update. Launch Call of Duty 4. If an update is available for the game, you will be prompted again. Accept the game update, and wait for the software to download to your system. Without this update, you cannot connect to the multi-player game.
Select the "Multi-player" option; then wait for Call of Duty 4 to connect to the Internet. Choose to connect to a private game, and select the party you wish to join. Make sure you type in the correct password. If you don't, you cannot join the private game.