While looking for a solution to this problem, I understand that "No flash video found" error means the youtube downloader doesn't work for them. The solution is simple as that for this reason! Try other downloader for u tube videos
1. Use icyvideo.com
http://www.icyvideo.com/ is a website designed for downloading videos on the web by entering the URL of the curresponding videos. (no software is required)
For downloading Youtube videos From icyvideo
1. Goto icyvideo
2. Copy the URL of the video page
3. Paste it in the URL box where it says "Enter the URL of the video page here"
4. Click "Download"
5. now you can see the download link there. Right-click the download link, choose "Save Link As." and save the video file.
If the problem still remains
2. Use internet download manager plugin
Download IDM from here and start save you tube to your hard disk