Adds a metabox to the menu admin page listing users so they can be more easily added as custom links to the WordPress Custom Menus
Check that the metabox is turned on under Screen Options. http://codex.wordpress.org/Administration_Screens#Screen_Options
Does the user you added actually have posts assigned to them? If they don't yet they don't have an author archive page in WordPress. I'm currently thinking about what to do for these scenarios, let me know what you think?
No, currently this plugin has no options and it shows user of the roles: super admin, administrator, editor, author & contributor. It does not list subscribers.
Maybe. Let me know what you want.