Thank you for getting back to us.
Yes, the Telerik.WinControls.UI.Design assembly is needed by our controls in order to behave correctly at design time. This assembly must be added to the Global Assembly Cache (GAC). I suggest removing all references from your project, reinstalling the RadControls for Winforms suite and adding the references again.
I hope this helps. Let us know, if you need further assistance.
the Telerik team