%vlc -vvv dvd:/dev/dvd
where /dev/dvd is the
name of your drive (put D: under Windows if
D is the letter of your DVD drive).
If you are running Linux, you must have write access to the device corresponding to your DVD drive. For that, you should be in the disk or cdrom group (look at the permissions in /dev). If you're not, add yourself to the group :
#adduser your_login disk_or_cdrom
and then restart your session.
If VLC doesn't work for you, try to spot an error message in the logs that are printed in the terminal.
Then, read the vlc FAQ and have a look at the Bugzilla to see if it is a known issue.
If you can't find out the problem, explain it in English in the mailing-list vlc@videolan.org and copy-paste in your mail the messages of the message window.