Error with Facebook app for Comments+

I created a facebook app as directed in the setup.... but I am getting the following error when trying to login to comment with FB:

Can't Load URL: The domain of this URL isn't included in the app's domains. To be able to load this URL, add all domains and subdomains of your app to the App Domains field in your app settings.

Not sure what I have done wrong! Support access is enabled... support appreciated. Thanks!