My recommendation is to use the RadEditor for MOSS
(in SharePoint 2007) or RadEditor for SharePoint 2010
(in SharePoint 2010) applications. They are especially designed to work in these environments and their File Browser dialogs use custom providers that are built to work with the SharePoint database.
If you still would like to use the editor in a custom SharePoint solution use the information provided in the following article: Using RadEditor in custom MOSS solutions
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