Unfortunately, YouTube videos, like most web videos now, are the FLV format, which MEP doesn't support.

There is a really nice video program called KMPLAYER (www.kmplayer.com) which supports virtually every video format, including FLV. It also has a "capture" function.  I've never used it myself, but apparently, it will capture the flv video and allow you to save it as a more common format, one that MEP does support.

You can download the video from YouTube by using the Mozzilla FireFox browser and installing the "Download Helper" extension.

Hope that helps,

Tim