I don't use RedHat either, but whatever file you want, it's definitely under /etc. The easiest way to find it is probably open up a command prompt and enter the following:
grep -r 'kdm' /etc/*
This command will search every file in every directory under /etc and for every file that contains the text "kdm" it will print out the file name and the line the text was found on. That should at least find you the file. If you need help editing it, you can just post the contents.