In the "Download" tab, click the "Downloaded" tab to see the downloaded videos on the right. Double click any item to play it with the built-in FLV player.
You can also use iTube HD Video Downloader to play your own FLV, MP4 and WebM videos on your hard drive. To do so, go to the "Transfer" tab and click the "+" icon to open your local folder, navigate to the directory where your FLV/MP4/WebM files are saved, and click "Open".
After that, right click on the video list and select "Play" to start playing the video.