In the Developer area of Facebook, you can enter multiple URLs. Make sure you have all combinations of your URL covered, including http and https. So, if your site can be reached at blah.site.com over both http and https, I'd recommend adding the following URLs:
http://blah.site.com/index.php?option=com_jfbconnect&task=authenticate.callback&provider=facebook
https://blah.site.com/index.php?option=com_jfbconnect&task=authenticate.callback&provider=facebook
http://site.com/index.php?option=com_jfbconnect&task=authenticate.callback&provider=facebook
https://blah.site.com/index.php?option=com_jfbconnect&task=authenticate.callback&provider=facebook
That goes for www and non-www or other URL variants that may work for your site.
Finally, please note that Facebook has disabled the Review process for permissions. You *should* still be able to get the manage_pages permission if you are trying to setup a Channel for a Facebook user that is the admin *of your app*, since admins can always grant any permission. If the Channel is for a normal user, and not an admin of your app, then you likely won't be able to get that permission from them until Facebook re-enables the review process.
I hope that helps get you going, but if not, let us know how we can help.
Thanks,
Alex