@Forester

Thank you for noticing my first mistake, typing the wrong command (knoppix_folder instead of knoppix_dir, I guess that was on subconscious level) but that didn't work either so I guess you are right and this issue can't be solved by a boot command.
I tried to hexedit the minirt24 file in the minirt24.gz archive and replace KNOPPIX_DIR="KNOPPIX" to KNOPPIX_DIR=“DSLINUX” , it worked.

I don't understand why VirtualBox scans it successfully but my dvd drive doesn't and all those commands I tried don't work:
dsl knoppix_dir=/dev/sr0/DSLINUX
dsl knoppix_dir=/dev/sr0/DSLINUX/
...
but since you mentioned that DSL is old anyway and I have Puppy linux, Tiny Core Linux and Slitaz on my multiboot dvd so it is not a problem so thank you for you advice and time!