In general, downloading videos from YouTube is not supported. There are two instances when downloading is offered.
Click to Buy option
We are currently testing an option with a small group of partners, giving them the ability to permit downloading of their videos downloads for free or for a small fee paid through Google Checkout. For more information, please visit our Help Center entry about partner video downloads.
Download your own videos to MP4
Sign into your account.
Click on the Account link in the top right hand corner of your screen.