Hi Stephen L,
The Light version of RadEditor for MOSS is not
suitable for SharePoint 2010. This version of the control has not been updated for a couple of years now. The only versions of RadEditor for SharePoint, which are designed to work in SP2010 are RadEditor for SharePoint 2010 6.x.x.
What I suggest you to do is download the free trial of RadControls for SP2010. It includes RadEditor for SP2010.
the Telerik team