When I try to play a DVD movie, I get a message about region codes.
All DVD movies contain a code so they play in only one of six regions in the world. This ensures that the format is supported by the display systems in that part of the world and also helps with piracy issues.
If you try to play a movie that has a code other than the one with which your player is set, you will see a dialog box enabling you to change the code for your player (in this case, the DVD drive in your Mac). If you choose to change the code, your DVD drive's code is changed. However, you can make this change only five times. At that point, the drive's code becomes permanent. I don't recommend that you change your drive's region code unless you are sure that most of the DVDs you will play in the future will have that code. If a code different from most of your movies becomes permanently set for your drive, you will be able to play only movies with that region code.
When I try to use a control, a NOT PERMITTED message appears onscreen.
The controls for a DVD movie can be activated only at specific times. For example, sometimes you can't scan forward through the warnings at the beginning of every DVD. The only solution is to wait until you are able to use the control.
When I try to view a DVD, all I see is a green screen.
This can happen for two reasons. One is that you are viewing a disc in the wrong region; the other is that DVD Player sometimes suffers from an occasional glitch. Try resizing the Viewer window with one of the commands on the Video menu. Most of the time, this clears any temporary problems DVD Player is having. If this doesn't work, the region code is the culprit.