Add search to primary nav menu Source http://my.studiopress.com/snippets/nav-extras/

functions.php

add_filter( 'wp_nav_menu_items', 'nav_menu_search', 10, 2 );
function nav_menu_search( $menu, $args ) {

if ( 'primary' !== $args->theme_location )
	return $menu;

ob_start();
get_search_form();
$search = ob_get_clean();
$menu  .= '<li class="right search">' . $search . '</li>';

return $menu;
}