If you bought a boxed version of EVE Online then you will have to proceed as follows.
1. Use the CD Key to create a new acct. ( yes, you will have to )
2. Create a character on that new acct.
3. Petition a character transfer ( trial acct char to new, full acct )
Note: If you are on trial and want to turn trial into full acct best way is to add a payment option/plan from the Acct Management
Buying a boxed version forces you to create a new acct ( with the CD-Key ) and then petition a character transfer.
There is no option to use a CD-Key to extend the trial acct ( which was created with a trial CD-Key )