Sorry, I never saw this post until now. It seems I don't always get notified of posts, for some reason.
In any case, the answer to your question is that free Add-Ons don't need to be registered (nor can they be). Registration is only for paid Add-Ons.
In recent releases of MapForge, you can shift-click the email field to have the app insert an "@" symbol. This is only required for users with international keyboards not set to English mode.