I wouldn't put a captive-ntfs-mounted drive in fstab. I'd mount it manually. A known bug listed on the Captive site says that writes aren't committed during machine shutdown. But that's your choice.*

Within KDE, bring up a terminal.
su
cd /etc
mcedit fstab
(make your changes and save it)

Then if you use the "Save KNOPPIX configuration" command off the Knoppix menu, and reload your saved configs the next time you reboot, Knoppix will use your modified fstab.

* Matter of fact, 90% of the time I have no luck getting files I edit on my NTFS partition to "stick". They edit and save OK, but I switch back to Windows and the changes haven't been committed. Hope your luck is better.