there are general problems with the PSN right now... mad rush on MW2 i guess.
When you get on the Playstation Store (Top Right) the is a Redeem Code Button, click on it and input your 12 digit code.
Mods are saying "If Possible, make any transactions out of peak times"
Not to sure where the code is on there, it's a mix of numbers and letters....
I have used the PSN cards and it has a silver thing u need to scratch off at the back to see the code
I have purchased one about 2 months ago...
You either have a newer version or a fake one by the sounds of it