If you have purchased a Game Time Card, you will need to apply it to your NC Account.
First, log in to your account here: https://login.ncsoft.com/login
Click on 'Apply a Code',
Enter the serial number printed on your Game Time Card, and click on ‘Activate’.
Additionally, game time cards can be exchanged for NCoin which is used by Lineage II and Aion. For more information about NCoin, and how to use your game time cards to buy NCoin, click here.