If you have the "Totally Unreal" cds, then you're out of luck for installing it under linux. I have those cds and I kept getting the same error message as you, at least that was until I read the Loki FAQ and realized that that cd was unsupported.
If that's your problem, I recommend going to Walmart and getting UT GOTY for $9.99. Problem solved.
I have the game of the year edition and have tried /mnt/cdrom also my cdrom automounts and I,ve checked in the konsole to make sure I thing that mandrake 9.0 just does something different.. I am going to load osx as well as Suse on my machine which already runs win doze98/me then Il'll try some more