Not sure, but isn't it "GameUI/UIContainer/FullscreenUpperContainer/MenuBar" ?
I am trying to move the menu bar in the UI editor, but I don't get it. Can't find where the anchors are.
I found too "<Frame type='Frame' name='GameUI/UIContainer/FullscreenUpperContainer/MenuBarFullscreenAnchor' file='GameUI'>", but no clue as it does not have any anchors. I am bored to search in all this to try to understand the mad spirits of blizz dev. To try to understand mine is enough.
Edit : what is sure, is that the path you indicate doesn't exist.