Follow these steps to redeem your PlayStation Network Card:

  1. Sign in to your PlayStation account in your browser
  2. Click on your Online ID at the top of the screen
  3. Select "Redeem Codes" from the drop-down menu
  4. Enter the PlayStation Network Card which you can find under My Withdrawals by clicking on "Show Code"
  5. The balance has now been added to your PlayStation account

Note: You can find the official instructions here.