How do I take screen shots on DVD movies and protected content?
This is deliberate from Apple and you will not be able to take screen shots using DVD player.

You can either use VLC and then a screen shot program (Cmd-3 and Cmd-4 works for me), or transcode your DVD to a QuickTime movie using Handbrake (which may take too long for you to find it interesting).  The latter will allow you to use QuickTime player which can do quite a bit.