You could use the markCurrentItem: true option and also some additional JS like this:
$(window).load(function() {
var $menu = $('#main-menu'),
obj = $menu.data('smartmenus');
// show current sub menu if menu is collapsible
if (obj.isCollapsible()) {
$menu.smartmenus('itemActivate', $menu.find('a.current').eq(-1));
}
});