I was having troubles displaying sub-menus on top navigation too, even though I had set child pages and parent pages, etc.
I used this WP information to solve the problem from the Administrator > Appearance > Menus
"You can also drag a menu item a little to the right to make it a submenu, to create menus with hierarchy. You'll see when the position of the drop target shifts over to portray the nested placement. When you have finished building your custom menu, make sure you click the Save Menu button."
I found the information here: http://codex.wordpress.org/Appearance_Menus_SubPanel
Under "Add Menu Items"
Thought it may help some other newbies like me, out there. :)