Your link is wrong. Make sure you typed it in correctly, and that it is, indeed, a Youtube link. We can't use any other video site for this feature.
The video you're trying to upload has already been uploaded (there should be a bright red notice that appears if this is the case). If this is the case, and this video is indeed YOUR video, contact a moderator to resolve the problem. Do not find the person who posted your video and flame them, as this will most definitely make the moderators much less inclined to actually help you out.
You will sometimes be unable to add videos to the library that were uploaded to Youtube within the past 24 hours. This is because Youtube doesn't fully publish the video for some time after the initial upload. Just be patient, not our fault.