How are you accessing the serial number window?
File -> New CSS Sculptor Page.
Copy and paste the serial number from your Download Center page, make sure not to copy any surrounding spaces. You can copy the entire string, and paste it into the first box of the serial input widow, it will automatically be broken down into the other input boxes.
If you are still having a problem, double check the title of the serial number window to make sure it is the CS Sculptor 3 serial window.
I see from your order history that you also have Sculptor v1, did you uninstall v1 from the extension manager before installing v3?