Its easy to make a multi-level drop down menu in Divi. First, go to Appearance > Menus in your WordPress dashboard and select the menu items you want to add to the menu.

To make a menu item a child of another menu item, simply drag it slightly to the right under the parent menu item. 

You will find that it stays there. Once it's on the right, it will appear in a drop down menu on the front-end of the website. So, in the screenshot posted above, the Test menu item will appear under Home in a drop down menu.

You can add more drop down levels in the same way.