Please go to http://www.sonyentertainmentnetwork.com. Sign into your account, and choose the Account tab at the top of the page. Next, you will choose Redeem Prepaid Cards from the menu on the left hand side. Note - codes need to be entered under a Master account (one which has a date of birth over the age of 18), or they will not work properly.
Answer I got from a rep. I didn't realize there were two types of websites to enter your account info to (psn and sonyentertainment)