I have replied to your previous forum post of the same issue. Here is my answer:
"You need to add one additional register directive in order for the RadChart control to work correctly as the Telerik.Charting namespace contains some core charting classes:
<%@ Register assembly="Telerik.Web.UI" namespace="Telerik.Charting" tagprefix="telerik" %>
Hope this helps. "
the Telerik team