I would try the simplest thing first
Right click on the dvd icon and go to properties, see what knoppix has selected for the device
If the device selected isn't the dvd then you can try and change it. Sometimes you may need to
create a new device ( right click on the desktop ) and select the correct drive as I explained above.

Under kernel 2.4 scsi emulation can sometimes cause problems. Under emulation cd drives are often /dev/scd0 or 1
But befroe getting into that see if creating a new device and selecting the correct drive gets you access to the drive.

"dmesg" at a terminal will provide most of the start up messages and/or hardware found during booting.

Good luck.