This is common editor for menus, context menus and tray menus.
When you first open menu editor, you'll see empty "virtual" item (it have dotted frame inside). This is just placeholder item, it will not generate any code and have not any properties in property editor. Click right mouse button on this "item" to bring up context menu:
Here you can select "Insert item" to create new actual menu item.
Newly created item can be converted into submenu (it's like regular item, but can hold another items).
Notes: