understanding wp_nav wp_dropdown

How do I create a wp_dropdown_menu using my current wp_nav_menu made in “Appearance -> Menus”?

My main nav has a couple of pages listed but now I want one of those pages to have a dropdown menu from it. Everything is setup and active the way I would like to have in “Appearance -> Menus” but the live preview just lists every page in a big bulk and doesn’t hide the dropdown.

I have searched the codex but I haven’t found anything about using wp_nav_menu with wp_dropdown_menu?