Sorry the manual dedicated to conversion is not ready yet, just the general how to from
http://www.downloadhelper.net/howto.phpThe new version is supposed to be more intuitive but we certainly have improvements to do based on users feedback.
From your remarks, i understand that you want to Download & Convert and you may have not noticed that the big + sign at the right of each entry in the VDH panel opens the list of possible actions, which includes Download & Convert (note that you can easily make this your default action by checking the parameter at the bottom of the actions panel).
You talk about the "Extension" parameter, so i understand you are trying to configure the conversion rules (they allow to automatically convert after a Download or Quick Download). Since you are not the only one being confused about this, we'll move the "Configure conversion rules" to the "Advanced" section.