First off, welcome to City of..., it sounds like you're on a trial account. You will need to purchase the full game first and upgrade the trial into a standard account, only then will it accept the time card.
Kismet is correct. There are two types of cards: The 30 day game card, and the 30 or 60 day game time card.
The game card can be used to create a new account, grants 30 days of game time, and comes with a nifty jet pack that lasts 30 days for each character on the account. This account will have access to City of Heroes and City of Villains, but not Going Rogue.
The game time cards can only be used to pay your susbscription fee. You have to have a full account (At least CoH/V, Going Rogue is not required) before you can use these cards.
@Roderick
The 30 day CoH game card is normally $20, creates an account, gives you a month and your characters get a jet pack for that month. At stores like BestBuy (and they may still be available at 7-11s) it is hanging with other MMO points cards that need to be validated at checkout. It's like buying a box version of the game, but without the box, or a DVD, or a manual (edit: or access to Praetoria or Going Rogue enabled content).
The Time Cards are in DVD cases and are good for NCSoft subscription based MMOs. The current case is for Aion, Lineage I+II and CoH.
Edit: You can also go to the NCSoft store and buy an account plus one month for only $9.99. But that won't get you access to Praetoria or any of Going Rogue enabled content. That requires the $29.99 Going Rogue - Complete Collection.
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I saw CoH, thought it was very good, and went out to purchase a 60-day game card.
I made an account (not using the game card 1st), made a character, took a look at the tutorial, and logged out to redeem my game card.
But when I enter the full serial number, it gives me this message:
"You can't use the supplied serial code.
The supplied serial code is valid, but it can not be used to create a new account and it can not be applied to any accounts you already have."
Any nice people out there to help me with this problem? Thank You.