How to correctly insert a search form into the top menu?

Hello. Could you please tell me how to correctly insert a search form into the top menu in the AT Subtheme theme, if it is not originally provided there?

Here's a screenshot with the block regions.

[URL=http://www.radikal.ru][IMG]http://i058.radikal.ru/1305/ef/104716d693d3.jpg[/IMG][/URL]

I read online that I need to insert the line page.tpl.php in the appropriate section: <?php print $search_box; ?>, but the thing is, for someone who doesn't know PHP, like me, this turned out to be quite complicated. When I tried to insert it in the right place, the search form still appeared either above or below the menu.

This is the part of the file page.tpl.php

<!-- Navigation elements -->

<?php print render($page['menu_bar']); ?>

<?php if ($primary_navigation): print $primary_navigation; endif; ?>

    <?php if ($secondary_navigation): print $secondary_navigation; endif; ?>