You have to buy the serial code, since you've already got the download all the code will do is tag your account as "paid." However, you must buy either the "game card" or an "edition" of the game (good versus evil, architect, or mac) to get that code. You don't have to buy a box, just head to NC's store, and choose one of those options. Since you're on a trial account make sure that when you have a paid serial number you apply it to the account you've already made, not a new one 
Please read my FEAR/Portal/HalfLife Fan Fiction!
Repurposed
ok thanks you helped alot, so it will give me a serial code to activate my account instanly right?
It should, yes, as long as your credit card/paypal goes through.
Please read my FEAR/Portal/HalfLife Fan Fiction!
Repurposed
The above is correct. I've purchased numerous CoH/CoV versions online and tied them to my account. It'll give you the option if you want to add the code to your existing trial account, or create an entirely new account. You may not see the full serial code, but it will apply it to your account.
(This is how mine looks)
Type Date Used Serial Code
Preorder Mar 11, 2004 XXXX-XXXX-XXXX-XXXX-GZEK
...
Retail May 26, 2009 XXXX-XXXX-XXXX-XXXX-98ZZ
I took several of them out for obvious reasons... but when I purchased the one from a couple days ago I saw it exactly like it is above.
I have downloaded CoH and made a trial account, now all i want to do is pay monthly to play the game properly now but is there some way to get or pay for a serial code without buying the box from a store because no stores in my country sell CoH?
please can someone help me