That's a special feature of the SCLogin module that's built-in. We're working on documentation for some of the hidden gems in the SCLogin module, but for now, here's how you make a logout button work in the SCLogin user menu:
* In the Joomla Menu Manager create a new "External URL" menu item in the user menu you're using SCLogin
* Set the title to whatever you want to show up as the text in the menu item
* Set the 'link' setting to "sclogout"
That's it!
I hope that helps, but if you have any questions, just let me know.
Thanks,
Alex