When you bought PSN card, there's no need for activation on the card side, it only contains code that is redeemable online ( similar to online voucher ). All you need to do is sign in on the PS3 with your PSN account, go to XMB account management and choose redeem code. Input the code written on the PSN card and the amount of money as stated on PSN card will added to you wallet.
I often bought PSN card codes online, all I did was input the code via redeem code option. If it does not work, there's only one possibility, the code has been used before or it's invalid.