The "Telerik.Web.Design.dll" typically gets installed in the GAC when you run the RadControls for ASP.NET AJAX installer. So, if you have these controls installed on your workstation, you'd have to navigate to "C:\Windows\Microsoft.NET\assembly\GAC_MSIL\Telerik.Web.Design" and retrieve the version information and use that to update the <add assembly="Telerik.Web.Design, Version=2010.1.309.35
, Culture=neutral, PublicKeyToken=121FAE78165BA3D4"/> entry in the application's "web.config" file.
documentation page. It provides a method for enabling Intellisense when working with RadControls for ASP.NET AJAX within your JS.
Hope this helps!