Comment 10 for bug 1248194

There are multiple issues underlying this.

First, gnome-mplayer (like many other video players) may look by default at /dev/dvd, but that device is no longer created by default.

You can restore this by (sudo) copying /lib/rules.d/60-cdrom_id.rules to /etc/udev/rules.d/60-cdrom_id.rules and changing the line beginning KERNEL="sr0" as follows:

KERNEL=="sr0", SYMLINK+="cdrom dvd", OPTIONS+="link_priority=-100"

Note that the concept of a /dev/dvd doesn't allow for multiple DVD drives. You have to use /dev/sr0, or make a suitable change to the above line if your preferred DVD drive is /dev/sr2, say.

Then, according to Debian bug report <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=777268>, the upstream distro on which Lubuntu is based used Mplayer2 in a version compiled without support for DVD menus (able to play dvd://1 for track 1 but not dvdnav://). gnome-mplayer offers 3 ways of opening a DVD with the File>Disc>... menu, but all rely on parsing the DVD menu structure and so fail. You can however add the VOB files from the DVD's VIDEO_TS directory to a playlist and play that.

gnome-mplayer is also unable to play a track as in dvd://1, presumably because it tries to use the menu structure in that case as well, even though the underlying mplayer can do so.

While the discussion of the above Debian bug indicates that this issue may be fixed in an upcoming LTS, you install SMPlayer (don't "Enable DVD menus (experimental)"). Not only does SMPlayer work better than gnome-mplayer, but in my view it's a much better thought-out and realised user interface that should be considered as a replacement for gnome-mplayer in future Lubuntu releases.