If you are outside the US you probably stumbled across the “Unfortunately this video is not available in your country” – error message.
Solution: use a web proxy to access YouTube.
Another reliable method is the following.
Just add “ss” inf front of youtube.com/…
Then click on the little video preview thumbnail on the right to watch the video (or choose one of the download options)
Other good proxy sites (simply copy and paste the video url into the proxy)