functions.php

<?php

add_filter( 'wp_nav_menu_items', 'be_subnav_search', 10, 2 );
/**
 * Search in Secondary Navigation
 * @author Bill Erickson
 * @link http://www.billerickson.net/genesis-wordpress-nav-menu-content/
 *
 * @param string $menu
 * @param array $args
 * @return string
 */
function be_subnav_search($menu, $args) {
	if ( 'secondary' !== $args['theme_location'] )
		return $menu;
	return $menu . '<li class="right">' . get_search_form( false ) . '</li>';
}