There are 2 types of cards:
- Game card = to open an game account, includes time
- Time card = to add time to an existing game account
From what you're saying, it seems you bought a Time Card to add 60 days of game time. Just hang on to it for later.
You need to either buy a Game card or a boxed game to upgrade your Trial account with. Check around, you can get a boxed game pretty cheap. Example: at Amazon. Each boxed game is the same game. The differences between them are just the extra perks (which you can also buy separately if you want).
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One thing : If, when creating your account via the upgrade, you use that time card when it asks you to set up billing (if you use a credit card at that point, you are not billed at that point) save that card. If something happens and you need to get back into your account and the password is lost/changed, one thing they ask for to verify you are the owner is a) what type of payment was the first payment (for credit cards, the first card CHARGED), and b) for credit cards, the last four digits. For time cards, the whole code.
Note, this is merely my paranoia speaking 
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And if you do get a box edition, DO NOT install it, just apply the code to your trial account. You already have the whole game. Installing from the DVD or CDs (depending how old of a box edition you get) will simply overwrite your current install and then 90+% of the game will need to be patched back up to the current version, which will take some time.
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I bought a 60-day game card from game stop when i entered the serial 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." came up, i thought game cards used to upgrade trial accounts