In the image we see the main tab for Buyers and the page tabs that drop down when you hover over it. The three items in that first level of the dropdown are shown at the arrows, and they are indented under the Buyers page.
Hovering your mouse over the How I Serve Buyers tab you see the second level of pages pop out (first green box). They are the ones in the second green box that are indented under the How I Serve Buyers page tab in the edit box. All you have to do to position pages properly for the dropdown menus is to drag them and indent them under their parent tab.