// Add search to main menu
function knaeckebrot_add_search_box ( $items, $args ) {
// only on primary menu
if( 'main-menu' === $args -> theme_location )
$items .= '<li class="menu-item menu-item-search">' . get_search_form( FALSE ) . '</li>';
return $items;
}
add_filter( 'wp_nav_menu_items', 'knaeckebrot_add_search_box', 10, 2 );