I'm so sorry on the delay in getting back to this. I just saw that there was an unread PM from you earlier this month that I missed.
I went to the site you linked to in the PM, and do see the popup login area.. however, that's the Gavick demo site. Is there a way to see your site? Even then, I can't really tell with just the front-end how to edit their template files.
My best suggestion would be to contact Gavick and see how to edit their login popup area. Once you know that, you just need to add the {JFBCLogin} tag where you want the button to show.
Alternatively, if their popup is simply loading a special module, you could try to load the SCLogin module in it's place.
I hope that helps, but if not, let me know.
Thanks,
Alex