I recommend to you to upgrade to the latest version of RadEditor for MOSS, which now is 5.4.1.
Now on the problem. You need to add the following code to the respective ConfigFile.xml or ListConfigFile.xml (for form scenarios such as list, wiki, blog), which are located in the Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\5.x.x.x__1f131a624888eeed\Resources folder.
More information on the document manager you can find in the following help article http://www.telerik.com/help/aspnet-ajax/document_manager.html
the Telerik team