When using VS 2008, both the Telerik.Web.UI.dll and
Telerik.Web.Design.dll files should reside either in the GAC or the bin
folder of your application. Do you have a reference to the
Telerik.Web.Design assembly in your project/GAC?
Please review the following blog post in order to avoid the
Also checkout the following links.
Error: Could not load file or assembly
Telerik Web Design Assembly