Is you site running SSL?
You can not use the plugin unless you have a valid certificate installed and running SSL.
Do you have settings correct in Facebook ?
If your website is called
then the Valid OAuth Redirect URIs is
and I also entered
All the other Facebook settings I have are:
Client OAuth Login: Yes
Web OAuth Login: Yes
Enforce HTTPS: Yes
Force Web OAuth Reauthentication: No
Embedded Browser OAuth Login: No
Use Strict Mode for Redirect URIs: Yes
Good luck !