Hello. I recently installed your plugin, and filled every request it asked me. When I first clicked on "Authorise application", a pop up came out and asked me to agree to what the application wanted to do. I agreed in everything, and then the settings page refreshed but authorisation was still needed. I tried taking authorisation away from facebook and reinstalling the plugin and the same happened.
I think my settings are ok, here are a couple of screenshots for you to check if I am doing it right.
EDIT: I tried forcing some variables to see if it worked, and I think the problem is $access_token is empty.