Instructions
Navigate to the PlayStation Network symbol from the PlayStation 3 home screen. Press the "X" button and then click the "Sign In" button to log in to your PlayStation Network account.
Navigate to the "PlayStation Store" from the system home screen. Press the "X" button to enter the store.
Click on the "Redeem Codes" icon found in the upper right hand corner of the window.
Enter the 12-digit code printed on the back or your PlayStation Network card into the three boxes in the redeem codes window to credit the funds to your account.
Browse the PlayStation Store and spend your funds.