How, and where exactly do you place this code? Are you creating a code behind of some of the existing forum controls/views?
Is it an option for you to change the RadEditor text from the Server side? You can do this by creating a custom control inheriting from the one having the editor, and then find the editor by using base.FindControl(). The custom control class should be placed in the App_Code directory and registered in Sitefinity as follows:
|<add name="Custom Forum Thread" section="Forums" type="Namespace.Class, App_Code" />
I hope this helps.
the Telerik team