In knoppix, the cdrom is mounted at /cdrom by default (not /mnt/cdrom). So if you want to mount the cd, it should be:-

mount /cdrom

Are you trying to play music cd? If yes, you may have to set the permission of /dev/cdrom to 777. I think the more correct method is to add the user to the audio group + others but I simply chmod 777 /dev/cdrom.

For music cd, vcd and dvd, there is no need to mount if you are using grip, kscd, xine, gmplayer......