php:

function jma_get_menu($args){
    $menu = isset($atts['menu']) ? $atts['menu'] : '';
    ob_start();
    wp_nav_menu(array(
        'menu' => $menu
    ) );
    return ob_get_clean();
}
add_shortcode('get_menu', 'jma_get_menu');

To insert the menu:

[get_menu menu="Main Menu"]