Registering at the Mac App Store

How do I register my copy of Mellel purchased at the Mac App Store?

You do not need to register a copy of Mellel purchased at the Mac App store.

You purchase Mellel at the Mac App Store using your Apple ID, and that ID is used to register your copy for you by Apple. Which is a good thing: when you need to retrieve a fresh copy of Mellel, you can use your Apple ID to download it, and it'll be registered for you.