See the answer here:
Dropdown Like Twenty-Eleven Theme Question
Be sure to use the updated code toward the bottom of my answer. It should be self explanatory, but if you have any issues let me know and I’ll help you out. Be sure to back up your files before changing them in case you break it.
How to Add the Menu After The Code is in Place
Once the menu is installed you should notice that the custom menu’s are now available in Appearance>Menu’s. Just type a menu name in the box provided, add some pages and posts, then drag the sub-menu pages below and to the right of the parent menu item and release. Be sure to save the menu when you’re finished.