Im assuming you want the audio? If so use http://www.youtube-mp3.org/ and paste the URL of the video (the address) into that box. It will then let you download the audio track to a folder on your computer. In iTunes, click on the toolbars under File until you find import file from library. At this point find the folder you saved the track in and click it. That should do it. If you wanted the actual video, let me know Amd I'll find a way.
To download a video from YouTube do this: Take the video address from the address bar like this "http://www.youtube.com/watch?v=iFca32_7YUU" Click on the address and type in the letters "ss" just sfter the "www." , Like this "http://www.ssyoutube.com/watch?v=iFca32_7YUU" Then hit the enter key or click on the arrow at the top right of the address bar.This takes you to a site where you can download the video in various formats for free...up to a certain size.