It looks as if you also need the xorg-dev metapackage and the many dependencies that it installs.

The safest way would be to install from the DVD where I would expect there to be these packages in compatible versions.

However to proceed from where you are now please first refer to
http://www.knoppix.net/wiki/HD_Insta...g_not_to_do_it
which explains, inter alia, the perils of upgrading from the various Debian repositories.

BTW I don't use installed Knoppix myself these days except as a reference system for the Forum, but I have found that the following worked safely for me ...

REPLACE your /etc/apt/sources.list with e.g. the following (you can replace uk here by your applicable country code for your nearest mirror):
deb http://ftp.uk.debian.org/debian/ etch main non-free contrib
deb http://security.debian.org/ etch/updates main contrib non-free
Then enter successively
Code:
apt-get update
apt-get install xorg-dev
NB Do NOT apt-get upgrade unless you don't mind risking breaking your system; whenever you need to install additional packages, just use apt-get install <package> on a case-by-case basis.
In fact it's a good idea to do
Code:
apt-get -s install <package>
first to see what the installation would do.