Add active class for wordpress menu

Add the following code in your functions.php file

add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);

function special_nav_class ($classes, $item) {
    if (in_array('current-menu-item', $classes) || in_array('current-menu-parent', $classes)){
        $classes[] = 'active ';
    }
    return $classes;
}