Hi ck,
Can you check to see if the Telerik skins is added to your project as a reference? It should be there called Telerik.Web.UI.Skins. This seems to be a recurring problem with a small percentage of users. Most were able to solve the issue by uninstalling & reinstalling RadControls for AJAX. Give it a try and see if this solves your problem.
http://www.telerik.com/community/forums/aspnet-ajax/general-discussions/no-skin-assembly-found.aspx
Hopefully this helps,
Master Chief