How to put site on maintenance mode

I wanted something simple, so I went looking for a plugin. I’ve tried at least 3 or 4 plugins now and not a single one will do anything! Every one installs fine and appears to go into maintenance mode from the backend…but no message on the front end. I’ve even tried different browsers, thinking maybe I can see it because I’m logged in as admin. No go. Still looks normal.

What am I doing wrong? The site is http://www.MissBrownSings.com