Creating a drop down menu in the Twentyten wordpress theme is quite easy once you know where to look.

When you are in the dashboard and are editing a page look for the Page Attributes section.

Under the heading "Parent" there is a drop down box.

Click on the drop down box and select the page that you want to be listed under in the main menu.

Next look for heading "order" in the page attributes section.

Fill in the box (1,2,3 etc.) this will organize your page listing order under the main menu.