Someone else recently had the same problem, and I wonder if there is a bug or something here, or some gremlins at work. I compared the other guy's file to my own, which I have in my sharing folder by the name of 'Dropdown hamburger menu', and which I have built following the same tutorial, and the only difference I found, was that I had given the container 'nav-bar' a width of 100%, while he had it at auto. Once I changed his file to reflect mine, everything worked. But, when he did it himself, it apparently did not work, so he ended up downloading my file.
So, now I found that your file also is lacking a width setting for the nav-bar, so I set it at 100%, and the menu was ok. Then, by accident I closed the file without saving, and when I reopened it, those 100% were gone, but the file was still working! So then I downloaded your file again, to a different location, opened it, and the menu didn't work. OK, so I popped those 100% width in for the nav-bar again, and it worked. Then I undid my changes, and it still worked!
Anyway, try setting the width on the nav-bar, and keep your fingers crossed!

If it doesn't work, you'll find my version of it in my sharing folder...