First-

Manually enter your code and tap the registration button, then go to another view in Informant and come back to the registration view where your code should now be accepted.

If this does not work.

Second-

1. In the Mac Finder, type SHIFT-CMD-G and then type in "~/Library/Group Containers/WZC4362K6S.com.fanaticsoftware.informant”.
2. Delete the Library folder.
3. Reboot the Mac because OS X caches the settings files and we need to force it to reset
4. Start Informant and enter the reg code

When you have to use the second set of steps... the problem always seems to be an OS X issue where OS X is not saving the preferences file and these steps will help you to write a new settings/preferences file