Mark,
When I try to login, the FB popup window shows "An error occurred. Please try again later.". That's almost always an indication of either of the following issues:
* Autotune hasn't been run recently. Mainly, your Application Domain or Application URL are incorrect. Re-run autotune and hit 'Refresh' in the top right of the FB App area. Make sure all your settings are as recommended.
* Your app is in sandbox mode. Go to the
FB Developers Area
and make sure the Sandbox is disabled.
Hopefully that will get you going, but if not, let us know.
Thanks,
Alex