Please note that version 5.0 or later of DNN uses the Professional provider of RadEditor, which is implementation of the DotNetNuke developers and it is not supported by Telerik. That is why my suggestion is to post your question in the DNN forums
or contact the support of DNN for help.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed