If you have created a custom menu then this may help.
1. You create a custom menu. http://en.support.wordpress.com/menus/
2. You add the Parent pages you want to display and arrange them in the order you want.
3. Then you add and drop and drag the subpages below the appropriate Parent pages, and when you have everything arranged in the menu that you want you save the menu.
4. Go to the “theme location” module at upper left on the menu page. Select your custom menu name from the pulldown labeled “primary location.” Click the save button in that module.
5. Refresh your browser so that it isn’t possibly pulling a cached page, and view see your new menu with Parent pages and dropdowns to sub-pages.
Troubleshooting – My menu isn’t appearing
Go to the custom menu page in the dashboard Apearance > Menus and in the “theme location” module at upper left select your custom menu from the pulldown and then click “save”.