you type
Code:
su -c "visudo"
hit enter and add what i wrot below to a new line below the one with knoppix.
Code:
yourusername ALL=NOPASSWD :ALL
you add text by hitting
i
then use arowkeys to move cursor to point you want (end of the knoppixline) then enter to start new line, there you just type your username and rest same as for line with knoppix and hit
Code:
shift+q then q and w folowed by enter
read here for explanation what that does.
http://floppix.com/vi.html

as for writing in fstab
su -c "kwrite /etc/fstab" should work as well.

there is no need to become knoppix, just that you have a different user than root is enough. otoh
su knoppix
in a shell will switch you to become the user knoppix (if you are not already).

sad fact is that installing knoppix is a bad introduction to linux, it is much harder than need be, ubuntu or fedora even Debian or Slackware are better as they are intended for hddinstall, in knoppix things are deliberatly "broken" to work well as live CD.
something you will have to fight...