with my linux native UT2004 it has a file called ut2004.run on the DVD.
to install it I open a terminal window and run the following command:
that starts an install script that asks some questions, i take the defaults and the game is installed to /usr/local/bin/ut2004.
you may have the same type of file on the CSS CD.
look for a file called css.run or something similar.
there should be a readme file or install document file on how to install the game on linux. It should have all the instructions you need.