My best suggestion would be to convert it through Handbrake app. Handbrake is an app that will convert it to the proper format for use in iTunes. Then place it in iTunes for playing. I am not aware of any way to play it from a portable DVD player.
There is a program called HandBrake that you can download from here. With HandBrake installed on a Mac computer, you can convert a DVD movie into digital video file and store it in iTunes. Once the digital video file is in iTunes, you can sync it to an iPad.