The most likely cause of this error message is that you are missing the following lines from your application web.config file.

<configuration>
	...
	<system.web>
		...
		<httpHandlers>
			<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
		</httpHandlers>
		...
	</system.web>
	...
</configuration>

If you are running under version 2.0 of the .Net Framework then you need to add the file system.web.extensions.dll to your application bin folder or download and install ASP.NET AJAX 1.0 extension.