Click "Manage Account" and sign in with your Sign-In ID(E-mail Address) and Password.
Click on "Redeem Prepaid Card".
Enter the code printed on the voucher or PlayStation Network Card and click "Continue".
If the code is valid, you will be presented with a description of what the code will deliver to your account. Press the "Redeem..." button to add the item or funds to your account.
TO REDEEM A CODE ON YOUR PS4 SYSTEM:
On the PS4 Dynamic Menu, go to the PlayStation Store.
Highlight "Redeem Codes" and press the X button.
Here you will be able to enter your code. Once the code is entered press the X button to continue.
Now you will see the item for redemption. Highlight the confirm option and press the X button.
A Message will appear that the code has been redeemed, press the X button on "OK".
The content will now show under the "Library" option.
For this example we redeemed an add-on for Assassins Creed. To find this content we will highlight Assassins Creed and press the X button.
Now highlight "Related Items" and scroll over to "My Add-Ons" and press the X button.
Here you can choose the items and download them to your console.