Our current SCLogin module is based off of the Joomla login module. If possible, I'd recommend using our SCLogin module instead of altering the Joomla Login module.
However, if you want to add our Login with Facebook button to the Joomla Login module, you need to copy the /modules/mod_login/tmpl/default.php file to /templates/<YOUR_TEMPLATE>/html/mod_login/default.php (creating any directories that don't exist). Then, edit that file you created and add the {JFBCLogin} tag wherever you want the button to display.
Hope that helps, but if you need more assistance, just let us know.
Thanks,
Alex