The SCLogin module is just that, a module. There's not a way built into the SCLogin module can make it work from a menu item. However, I know some sites have plugins that can let it work. I honestly don't know how, but what would need to be done is:
* Put the SCLogin module in a position on the page
* Hide the normal login button area that the SCLogin module would normally show using CSS
* Create a custom menu item that has a link like:
<a href="#login-modal" role="button" data-toggle="modal">Log in</a>
any link with the href like the code above should make the SCLogin module show, if it's published on the page somewhere.
I hope that helps get you started,
Alex