There needs to be a DVD disk in the drive. You can't mount an empty dvd drive. You can't mount a blank DVD. There has to be a file system on it.
You can mount a data or video DVD.
mount: no medium found on /dev/sr0
Looks like there is no file system in the drive or you are mounting the wrong device node.
If your DVD is ok then look for the drive.
Gnome probably has some tools to tell you where your drives are.
I thought that ubuntu was suppose to auto mount drives. What is it using? HAL, udev?
should find your optical drives too