Please be sure you are copying and pasting just the serial number itself -- sometimes when you copy/paste text, an extra space or line break finds its way in at the end of the serial number.
Also, please make sure you are registering with your first and last name and/or email address exactly as they appear on your MacUpdate user account.
If you haven't received your serial number yet from your purchase, go to your Purchases page. On this page, you should be able to scroll and find the latest serial number for each app that you've bought from MacUpdate.
Didn't find what you're looking for? Feel free to contact us directly.