how do you register

[peepeemd]

i bought the game
i got a registration number

Now, how do i input the number to unlock the demo.

I must be some kind of moron -- but I can't figure this out.

thanks in advance

[T¹]

Don't worry, you're not a moron! The reg code doesn't unlock the demo, but is used during the installation of the full game. After purchasing Snails, did you download the full game installer, in addition to writing your registration code down? When you run this installer, it will prompt you for your reg code.
BTW If, for some reason, you didn't download the full version from the order acceptance page, you should be able to re-login, confirm your details and download it.