It won't let me either, through the PSN Store or the Account Management screen. I get "an error has occured" in the PSN store and in the account management screen "the funds could not be added to your wallet"
I just called Playstation help line, they said either the card was never activated in store, the billing address is wrong on the account, or that the code was just bad. I have to send in an email to get another code sent in to me. Just call the help line, it doesn't take long and they will tell you what to do.