Here is what I did:

root@1[knoppix]# cd ../..
root@1[/]# rmdir cdrom
root@1[/]# mkdir cdrom
root@1[/]# cd cdrom
root@1[cdrom]# dir
root@1[cdrom]# ln -sf sr0 /dev/cdrom
root@1[cdrom]# ls -la /dev/cdrom
total 40
drwxr-xr-x 2 root root 4096 Sep 2 22:12 .
drwxr-xr-x 13 root root 36864 Sep 2 22:09 ..
lrwxrwxrwx 1 root root 3 Sep 2 22:12 sr0 -> sr0

What else should be done?

After doing the lib thing the dvd finally can play dvds. Thank you so much. But now I can't paly audio cds with kaffiene and get these error messages:

The source can't be read.
Maybe you don't have enough rights for this, or source doesn't contain data (e.g: no disc in drive). (/dev/dvd) and in kaffiene window : Error: no plugin found to handle this resource.
Is this another lib thing?