YouTube video downloading is no longer supported in order to maintain compliance with YouTube Terms of Service posted at <http://www.youtube.com/t/terms>:

"... You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content. You shall not copy, reproduce, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any Content for any other purposes without the prior written consent of YouTube or the respective licensors of the Content. YouTube and its licensors reserve all rights not expressly granted in and to the Service and the Content."

http://getbookmarklet.com/yt provides a bookmarklet that may be of interest to you. Tap here to open the link directly in Downloads, or, if you are using Downloads Lite, tap here instead. UPDATE 12/09/15: YouTube has changed their mobile site yet again. Existing users must reinstall the bookmarklet for it to work correctly.