I already answered your previous post, but for your convenience I'll place the solution here again:
Generally speaking, this error occurs when your site is not properly configured to use AJAX. To correct it you need to ajaxify your site.
The "Prometheus" suite is based on MS AJAX Framework and in order to use the controls which it contains you must have the ASP.NET Extensions downloaded - if you don't have them, you can get the distribution from here
In order to do this, you can open a new AJAXEnabled WebSite and start building your application in it.
As an alternative, to ajaxify an existing web site, you can open a new AJAXEnabled one, copy its web.config file and update your existing project's one with the new web.config.
If you continue experiencing problems, please don't hesitate to contact us.
the Telerik team