To connect or link Facebook app to your Facebook page, follow the simple information provided along with images in this article. Linking your Facebook application to your existing page is very straightforward. Just there is no enough information provided on Facebook documentation or elsewhere.
When you setup App on Facebook and try to make login with Facebook available in your script, everything seems fine but when someone attempt to login with a Facebook account he/she gets either of these errors from Facebook: App Not Setup: This app is still in development mode, and you don’t have access to it. Switch to a registered test user or ask an app admin for permissions. The developers of this app have not set up this app properly for Facebook Login.