Hello Andy,
I used this code to add class “simplemodal-login”. It may help you
//Conditional classes
add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);
function special_nav_class($classes, $item){
//replace "login with your login linkt title[must in lower case]"
if(strpos(strtolower($item->title), 'login') !== false){$classes[] = 'simplemodal-login';}
return $classes;
}