assuming you are running knoppix 5.1.1, and reading from a windows ntfs filesystem:

mount -t ntfs-3g /dev/<whatever> /media/<something>

you may need to create /media/something
/dev/<whatever> should be in the "shadow" image on the desktop, otherwise look at the output of "fdisk -l"

a thought - are you trying to mount as root or as a normal user?