file is only copied to the ControlTemplates folder if you enable the "Use RadEditor to edit HTML Fields" feature from the Site Features page. The template file overrides SharePoint's RichHtmlField
template and displays RadEditor instead of the original content editor. The RichHtmlField template is not available in WSS v3, so enabling the HTML Fields feature has no effect in this case (apart from the exceptions you see in the application log).
It seems that my colleague misled you when he directed you to this page - http://www.telerik.com/help/aspnet-ajax/using-radeditor-in-web-content-management-scenario.html
. The page describes how to use the editor in a WCM scenario, which is also not available on a WSS v3 only site. I apologize for the mix-up.
When you are using the RadEditor for MOSS in a WSS only environment, you only need to activate the List Items feature - http://www.telerik.com/help/aspnet-ajax/using-radeditor-in-list-items.html
. This feature uses a different template file - TelerikRadEditorText.ascx and overrides the RichTextField
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