I think this is because Knoppix is based on Debian Stable which has no official Nim package.
Do the following in a terminal to download the package: `wget http://http.us.debian.org/debian/pool/main/n/nim/nim_0.15.2-2_i386.deb` And then after running that command do the following to install the downloaded package: `sudo dpkg -i nim*.deb` >From there on it should be easy.
