Question: how to download youtube videos using xdm?
Answer: Launch XDM MediaGrabber. You may run XDM MediaGrabber from XDM main menu by selecting "MediaGrabber" menu item or click on Grabber button from XDM toolbar.
Play media with your browser (if a media file has been playing before XDM's launch, refresh page) ... 
Click Download button in XDM MediaGrabber.