To create submenus you should choose the button at first. Let's add 6 subitems for the "Products" button for example. To add submenus you should click "Add submenu" button situated on the button builder Toolbar once.
You'll see that the "Products" button has 1 subitem now.
To add more subitems click "Add item" button on the Tollbar. The "Products" button has 5 subitems now.
To add the next level of subitems click "Add submenu" button once and then click "Add item" button.