How can I make the menu always open on desktops?

ShiftNav is an off-canvas menu, and as such it is designed only to display when toggled open by the user.

If you would like to force the menu to be open at all times above a certain viewport width, you can add this CSS:

@media screen and (min-width:960px){
    #shiftnav-main {
        -webkit-transform: translateX(0);
        -moz-transform: translateX(0);
        -ms-transform: translateX(0);
        -o-transform: translateX(0);
        transform: translateX(0);
        visibility:visible;
        opacity:1;
    }
}

You can change 960 in the media query to your desired breakpoint.