I had the same problem. Then my wife (who is Chinese) showed me that one must download media player apps using TVpad's app manager. The app for video playback is called Mvod. (For audio, it is called Mplayer. Apparently, you need a separate app to play audio files - I have not tried Mplayer myself.)
Once you start the app and point it to your USB drive, it will scan the drive and show you a list of files that can be played. If your drive has many videos, the scanning process will take quite a bit of time, which is really annoying. I do not think it is possible to disable the scanning process and instead ask the app to show you the drive's folder structure, so that you can navigate to the video yourself without TVpad wasting time with figuring out a complete list of video files on the drive.