I am assuming that this is the article, which you used http://www.telerik.com/help/aspnet-ajax/radeditor-in-custom-moss-solutions.html
I can see that there is something missing in the CreateChildControls() method.
Here is how the method should look like.
|protected override void CreateChildControls()
| contentEditor = new MOSSRadEditor();
| contentEditor.Content = "Enter content...";
| contentEditor.ID = "MOSSRadEditor1";
I hope this helps.
the Telerik team