I already tried to install a .deb and a .rpm file with KPackage, but both cancelled with an error.
We really need to know the error!

Are you using a HD install? If so, please use apt-get to install packages.

As root do

apt-get update && apt-get install dia gsfonts-x11


Using .rpms (or source code) should only be done in an extreme emergency: it'll make it impossible for apt to upgrade that package.

While what td says about Kanotix is true, much of the peril of upgrading/updating Knoppix can be avoided if you point your sources.list to unstable and follow the instructions at the very top of the original /etc/apt/sources.list.