How does one add additional items to NSMenu in Interface Builder?

I'm using Xcode for the first time. I've been following a tutorial, and I'm completely stumped by something that's surely trivial. I've dragged a "Menu" from the Library into my "MainMenu.xib" window. Double-clicking this menu makes it appear.

No trouble thus far. Editing the three items is intuitive, as is deleting items. How on earth do I add an item to this menu, though?

You want to drag an NSMenuItem from the library palette (View > Show Library) onto the menu:

You can add submenus and separators similarly.