With a bit of luck, a new icon should popup on your desktop when connecting the drive, and you can then mount by right-clicking. If this doesn't work, try booting with the drive already active, knoppix should then notice it and create an entry for it in /etc/fstab (and a corresponding desktop icon). If you insist on using "mount", see "man mount" (or "#mount" in the konqueror address bar). Hint: you need to provide the device entry and mount point; the filesystem type is usually guessed correctly. Something like
$ sudo mount /dev/sda1 /media/sda1
The mount point (/mnt/sda1 in older releases) should have been created for you but there's a slight chance you'd need to md it yourself.
Note that only with the most recent KNOPPIX releases has it become safe to write to ntfs; you'd then use
$ sudo mount -t ntfs-3g /dev/sda1 /media/sda1