You have to be logged onto the computer as an administrator when the software is being registered. The registration information is saved in a protected area which requires administrator privileges.
When you are logged on with a regular user account, the info never gets saved and you are asked to enter it again the next time the program is launched.
You just need to log on as an admin, enter your serial number, then Quit it and log back on with your normal user account.
If you are still having trouble, and you are on a Mac, see the Macintosh FAQs here.