The free MOSS version of RadEditor can only be used in the three scenarios, which are described in the installation docs.
If you want to add the editor as a field in a MOSS page, you can use the Telerik.SharePoint.FieldEditor.RadHtmlField object (WCM scenario).
if you want to use the regular ASP.NET version of the editor, you will have to download either a trial or developer build from our site and replace the RadEditor.Net2.dll and RadSpell.Net2.dll assemblies with the regular ones from the downloaded version. You will also need to update the scripts folder with the regular version. Extract the NET2\Editor\Scripts\7_0_2\ folder in
c:\Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\188.8.131.52__1f131a624888eeed\RadControls\Editor\Scripts\7_0_2\ on your MOSS machine. Choose to replace files when prompted.
In addition, we are going to release a new version of the MOSS editor, which will not be free, but you will be able to use it in regular ASP.NET forms as well as MOSS. The new version should be released tomorrow (assuming that testing does not uncover any serious bugs).