In order for a button to show up in a submenu of a top nav page, it must be listed as a subpage of that top nav page. So in your example, About Us will need to be listed as a subpage of your homepage. In the page settings of "About Us", you can fill in the "This page is a subpage of" field with the slug of your homepage. You will also need to make sure that the "Show in top nav" box is checked so that it shows up in the subnav menu. There is no way to have a page show up in the submenu of a top nav button if it is not a subpage.

If you'd like to have the same page show up in multiple locations or submenus, consider using a redirect page to create multliple instances of the same page.