First make sure that you upgrade to RadEditor for SharePoint 2010 6.x. This is the control, which is designed for SP2010 (5.x is designed for MOSS) and its latest version is 6.1.6. You can find the upgrade instructions in the following online help article: Upgrading to a newer version of RadEditor for SharePoint 2010
. Points 7 and 8 of the provided article regard your problem.
All the best,
the Telerik team