Make sure you have installed ASP.NET AJAX. Info can be found at http://ajax.asp.net/docs/InstallingASPNETAJAX.aspx. If your web application is not using ASP.NET AJAX you need to configure it to do so. Detailed instructions can be found at http://ajax.asp.net/docs/ConfiguringASPNETAJAX.aspx (Look for the topic called Adding ASP.NET AJAX Configuration Elements to an Existing Web Site.).