The theme is based on the Twenty Fourteen theme, and like the base theme, a hamburger menu is supposed to appear when the site is displayed on a mobile device. The problem is that the icon for the hamburger menu is black, and the background for the navigation bar is also black, so you can’t see it. It’s actually there on the right side of that black bar. If you tap/click on it, a drop-down menu will slide down, which is how the mobile menu works.
So the solution is to either change the color of the hamburger menu icon, or change the background color of the navigation menu bar.