When you add a new page, it is automatically placed into the menu structure but it will not show up in the dropdown menus until the cache is cleared. At the top of the page there is an admin menu down arrow. If you have that arrow, clicking it will show options, including "Clear all caches". Clicking that will do it and you don't need to do anything on the page that appears.

If you don't have the admin menu arrow, click this link to clear the cache. The new page should now appear in the menu.