I have tried to modify the example from tutorial5 (database relations) by adding a description field(HTMLText) to the Module class.
Then, in the function getCMSFields_forPopup() I added the line:
$fields->push( new HTMLEditorField( 'description' ) );
I have seen a similar discussion [url=http://www.silverstripe.org/archive/show/40582/]here[/url]
I really like SilverStripe, but this kind of problem is a show stopper for me.