You need to install a maintenance plugin like this one: http://wordpress.org/plugins/wp-maintenance-mode/ and then activate the plugin and go to it’s settings to turn it on.
But I don’t know how you make the enfold ‘maintenance mode’ page the page that displays, which I think is what you’re really asking. Maybe someone knows of another maintenance mode plugin that allows you to set the page that is displayed?
Create a page, go to Layout > select No Sidebar. Don’t display the header, footer widgets and socket. Add a Text Block or whatever it is you want to add on your Maintenance Mode page. Publish the page. Go to Enfold > Theme Options > Frontpage Settings, select the maintenance page.