Our modal popup in the SCLogin module brings up a window with the login form for Joomla credentials and social network login buttons. To do this, you should simply have to set the "Login View Settings" option in the module parameters to Modal Popup. When you click the Register buttons in SCLogin, we're simply redirecting the registration to the appropriate extension for the "Registration Component" option. We do not provide the ability to launch this in the modal popup currently. Likewise, if you use the social network login buttons, we'll be launching the JFBConnect component, also not in the modal popup but a standard Joomla view.
-Melissa