The first time you activate a RadEditor feature, you must be logged as an Administrator user. This is required, because the editor needs to update the default list editing template by copying a file - from \TEMPLATE\FEATURES\RadEditorFeature\RadEditorList.ascx to \TEMPLATE\CONTROLTEMPLATES\RadEditorList.ascx.
The file is 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 file. Once the file is copied, any user can activate/deactivate the RadEditor features.
Also make sure that you execute iisreset command after you activate the features for the first time.
the Telerik team