The "Register with Facebook" link doesn't work

I just installed the "Ultimate Facebook" plugin, but the "register with facebook" link just redirects me to my main page (http://www.pcsteps.gr), without registering me or logging me in.

http://www.pcsteps.gr/wp-login.php?action=register

I have followed this thread to the letter after the installation of the plugin: https://premium.wpmudev.org/forums/topic/how-to-make-a-facebook-app-for-your-site

I also went through all the settings and:

-Granted extended permissions
-Activated the "Allow users to register with Facebook" option

But it doesn't seem to work.