I can't connect to the above mentioned website so I don't know how you configured your system (I get a 404 Not Found error), but you get that error because Debian doesn't know what to do with /mnt/dvd. Do you have the directory /mnt/dvd? if so then you have to edit your /etc/fstab or/etc/mtab to be able to mount the DVD with "mount /mnt/dvd". Check out the following link for information regarding the fstab file:
http://www.tuxfiles.org/linuxhelp/fstab.html#what
If you are trying to watch DVDs, then you do not have to mount the drive. You simply put in the DVD, open the DVD player (I personally like Xine), and play the DVD. Same for audio CDs. If you want to look at a data CD, you can try this (as root, I think you can only mount things as root):
# mount -t iso9660 /dev/dvd /mnt/dvd
the "/dev/dvd" part can be replaced with "/dev/hdx" where x is the letter of your drive (my DVD player is the Secondary Master so it is /dev/hdc). I've never had to mount a drive to read a data DVD so I'm not sure if it is the same as a data CD.