Arthur, just for grins, Go up to your Apple and shut your computer clear off
Shut Down...Wait 4 or 5 minutes now restart.. Go directly to your Mac HD and open it . Next click on Applications.. when it opens go to DVD Player and double click and open it.. After it opens to the Black Screen. now insert a Commecial DVD Movie..AND DON'T TOUCH ANYTHING..
It will take a minute to read the Security, and Encryption .
It should start to play your movie automaticly...
I have found this to work when your DVD's don't mount automaticly..
You should be able to just put a DVD in and it will mount and start... But occasionlly Not..
If this works for you you don't have a problem,
If not then you do have a problem...Cheers...Don