If you have a PlayStation Code, follow these step-by-step instructions to use it at PlayStation Store on your Playstation:

Go to Redeem Codes.
Follow the on-screen instructions to enter the voucher code details. (Once complete, select Continue.)
Select Confirm.
Select Download. (Once complete, the item will be added to your Download List.)
Access your content. (Go to the relevant category on the XMB™ Menu to start playing your content.)