acces to a registred page ... doesn't propose the FB connect

I'm not sure what you're going for. I'm assuming you mean that the user is being redirected to the "Login" page when they try to access a page marked as Registered.

To do add the Login with Facebook button, there's a few options:
* Add the login button the the /components/com_user/views/login/tmpl/default_login.php page. For more information on the code to add the login button, check out:
www.sourcecoast.com/extensions/jfbconnec...estions#login-button

You should do this using a template override. For more information on that, check out:
www.sourcecoast.com/extensions/troublesh...ng-joomla-extensions

* Alternatively, you can add the JFBCLogin module to your Login page. This would be in addition to the login box that's already there, which wouldn't be optimal, but it's an option. We recommend the Advanced Module Manager which will let you easily add modules to very specific pages. You can get the advanced module manager below:
www.nonumber.nl/extensions/advancedmodulemanager

Hope this helps gives you an idea of what to do. If you need more help or more information, don't hesitate to ask!