The code can be found here. It would need to be in a module that can execute PHP code. I don't believe mod_custom can do this. There's a module called mod_php (
download page, v1.0.0beta2
). This lets you easily insert custom HTML, javascript and PHP.
Code to add for the Login Button:
www.sourcecoast.com/forums/topic?id=1227
In the very near future, we'll be adding {FBLogin} tag or something similar so that you won't need to mess with PHP and can very easily insert the login button anywhere.