Hello Speed,
Make sure that you have all necessary assemblies in the GAC or in the bin folder of your SharePoint web application. You need to have
Telerik.Web.UI.dll,
Telerik.Web.Design.dll,
Telerik.Charting.dll, and
System.Web.Extensions.dll. If one of these files fails to load, MOSS will show the error message you encountered.
Sincerely yours,
Peter
the Telerik team