Instructions
Turn on your PlayStation 3.
Choose "PlayStation Network." From this screen, select "Sign Up for PlayStation Network."
Select "Sub Account."
Input an email address to be used for the account, and then enter a password, a password recovery question and the user ID you want to use for your "Modern Warfare 2" split screen mode. This new account will be created on your console.
Select "Sign In" from the PlayStation Network menu. Sign in with the information for the sub account you just created instead of your original PlayStation account. The user ID for the sub account will be the new ID in the "Modern Warfare 2" split screen mode.