That error is usually encountered when the application has not been set up correctly, specifically the redirect URI setting. Could you verify this setting is correct? Additionally, please ensure that you've put the correct API key into JFBConnect - there are two API keys (one for authentication and one for widgets).
What is the exact URL of the page that you're on when you hit the Meetup login button? I would like to see it in action.
-Melissa
Meetup connection not working
