Menu not showing on mobile devices

Hi guys, I'm using your Chicago theme and am very happy with it! I found an issue with the menu in responsive mode (on mobile devices). There is shown a button "MENU", but nothing happens if you tick it. Not on phones, not on tablets. Please see my site here: www.kwf-amstelveen.nl. Would be great if you could help me out and fix this. Thx! Greetings from Amsterdam, Joris