Please make sure you are typing in the registration code. It is pretty common to accidentally type in the product # or order #. Scan down your order email to the line that says "Your registration code is: " and the number right after that is your registration code!
Also, make sure that you've got the correct converter program. We've got two different converters and your code will only work with one of them. Audio Converter Pro is here.
MP3 to Wave Converter PLUS is here
You can upgrade from "MP3 to Wave Converter PLUS" to "Audio Converter Pro" for $9.95: here are the details.