I'm pretty sure the page you linked can be found in the following file:
/components/com_community/templates/<YOUR_JS_TEMPLATE>/register.index.php
Ideally, you should create a template override for that file by copying it to /templates/<YOUR_JOOMLA_TEMPLATE>/html/com_community.
Once there, edit it, and you can add the { JFBCLogin} (no spaces) tag anywhere you want the Login button to appear. This will allow your users to bypass the registration form.
Hope that helps, but if not, let us know. Good luck!