In the menu bar, there is an NSMenuItem which contains an NSMenu. Both have a "Title" property. In Interface Builder, what you see in the menu bar is the title of the NSMenuItem. In the application, what you see is the title of the NSMenu. If the NSMenu's title is not set, then you'll just see an empty string, which is what you've shown in your screenshot.
Click on the Menu item in Interface Builder so that the attached menu shows up. You can then set the title.
Note that in Interface Builder, if you drag a Menu Item to the menu bar, you get an NSMenuItem without any NSMenu attached. I suspect that's what you've done. You can drag a Menu on top of the NSMenuItem to add a menu to it, and then you can set the menu's title correctly. If instead of dragging a Menu Item, however, you drag a Submenu Item, then the attached NSMenu is already present, and it automatically updates the title when you change the text in the menu bar.