How to add search box in primary menu

I've just figured it out a little by put a code below in page.tpl.php

if (isset($top_menu )) { print theme('links', $top_menu , array('class' =>'links', 'id' => 'main-menu')) print $search_box }

and CSS

#search-theme-form {float: right;}

But I cannot centered it rightly. (refer attachment)
Someone please help me to align my search box correctly.