.toggle() has been removed from version 1.9 and you are using jQuery v1.10.2
in your site.
It works in fiddle as ---> fiddle has jQuery v1.8.3
.
Use this code
$(document).ready(function () {
$("#button").click(function () {
var txt = ($(this).text() == 'Hide Content') ? 'show Content' : 'Hide Content';
$(this).text(txt);
$("#hidden_content").slideToggle("slow");
});
});