<?php
/* Primary menu */
if(function_exists('wp_nav_menu')
) {
    wp_nav_menu(array(
        'theme_location' => 'primary',
        'container' => '',
        'container_class' => '',
        'container_id' => '',
        'menu_id' => 'main-menu',
        'menu_class' => 'main-nav',
        'fallback_cb' => false,
        'walker' => new Multilevel_Menu()
    ));
}