Just a few random thoughts:
1. Ensure that the <telerik:RadEditor> tag has the OnClientLoad property:
|<telerik:radeditor runat="server" ID="RadEditor1" SkinID="DefaultSetOfTools" OnClientLoad="OnClientLoad">
| <strong><br /> Switch to Html mode to run the content filter.<br /> You will see that the casing is changed to upper case.</strong>
2. Make sure the External Template is mapped and in place. Georgi linked the instructions above.
3. Make sure the web application gets restarted after the Telerik.Sitefinity.Configuration.ControlsConfig.xml
file is put into place or changed. This file is only processed during Application_Start. Changes won't be seen until the web application is restarted.
I'm not sure if any of this helps, but those are my stabs in the dark. ;) Best of luck.