Ok, first include jQuery library in the <head>
section of your document e.g.
Google CDN version
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
or download the compressed, production jQuery 1.11.0 and put it in your root folder then include it like this
<script src="jquery-1.11.0.min.js"></script>
then add below script just before the </body>
tag
<script>
(function($) {
$('.search_button').click(function() {
if($('.search_function').css('display') === 'none') {
$('.search_function').slideToggle(350);
}
});
})(jQuery);
</script>
Here's a FIDDLE