Ah, the good old days of serial numbers for Apple software:
Not all iWork ’05 serial numbers will work with Pages 1.0 and Keynote 2.0. If you’re having difficulty entering your serial number, updating the software may help.
The first time you open Keynote or Pages, you are prompted to enter a serial number. The screen should also contain a “Buy Now” button. If this button isn’t present, you may need to update your software before you can enter your serial number.