Hi Rob,
Thank you for writing in, the hamburger menu is there in the upper-right area of the header, but its white on a white background so it is not visible. To address this you can either change the Navbar Background color under Theme Options > Ethos. Or you can change the color of the hamburger menu using the following custom CSS.
.x-btn-navbar,
.x-btn-navbar:hover,
.x-btn-navbar.collapsed,
.x-btn-navbar.collapsed:hover {
color: #003e85;
}
You can add this under Theme Options > CSS, free feel to change the color value ( #003e85)
Hope it helps,
Cheers!