If you purchased the full version of Fade In Professional Screenwriting Software and it requires you to enter your username and registration key, you'll find that information in the email sent to you after purchasing.
You can enter your username and registration key under Help > Register.
Be sure that you download and install the full version from your personalized download link (also provided to you via email). You cannot enter your registration information in the free downloadable demo.
(Mac users may want to make sure that there isn't an extra copy of the demo version floating around. You can search for it in Finder by searching for Applications named "Fade In".)
If you find that you are unable to to enter your username and registration key, it's possible that you have inadvertently downloaded the demo version. Please make sure you've downloaded the full version from your personalized download link.
Also please make sure you've entered your username and registration key exactly as given in the email. Copying and pasting from the email may help ensure this. If, for instance, the registration does not match a valid format/pattern, the Register button will remain grayed out:
Tags: full version, registration