» Download ClickToFlash 2.7 for Safari 5.0.1 or later

1. Drag and Drop ClickToFlash 2.7.safariextz to Safari Window.
2. Click install.
3. Go to tab 'Shortcut menu' > Check a 'Download Video' check box.
4. Play YouTube video > right click on the video > select Download Video. 

If you can't download, click here