How to redeem a PSN code.
Online
Step 1: On a web browser, Sign in to your PlayStation Network
Step 2: In the Account section on the left, select Redeem Codes
Step 3: Enter your 12-digit voucher code or prepaid PlayStation card number, and select Next
Console
Step 1: From the Playstation Menu, go to the PlayStation Store.
Step 2: Select Redeem Codes on the left hand side menu
Step 3: Enter the code and select Continue
Step 4: The content will be automatically applied to your account, available for immediate download, or available in your Library > Purchased section