There's a few ways to add the button. I'm not completely familiar with the structure of the hellome module, but here's 2 options for you:
* Edit the Hellome template file, which is likely at /modules/mod_hellome/tmpl/default.php. Wherever you want the Login with Facebook button, add the {JFBCLogin} tag and the button will appear.
* Create a new SCLogin module that shows directly under the HelloMe module. Set the parameters so that the forms and other links don't show, just the Login with Facebook button. Publish it and set it to only show for guests.
Hope that helps, but if not, let us know!
Thanks,
Alex