Hi Nishat Mazhar,
The control requires different settings in the web.config file, depending on the version of IIS it's intended to be run at.
Please see the following help article to how configure the settings.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed