Please check if the first time you activate a RadEditor feature, you are logged as an Administrator user? This is required, because the editor needs to update the default editing templates by copying files to \TEMPLATE\CONTROLTEMPLATES\
The files are copied by the IIS worker process, which in turn is impersonating the currently logged in user. That is why you need to be logged in as an Administrator to be able to copy the files. Once the files are copied, any user can activate/deactivate the RadEditor features.
the Telerik team