I was trying to install ut2004 on ubuntu and kept getting the message:
"please mount the unreal tournament play disc cdrom"
I think because ubuntu auto-mounts volumes, it mounts them in multiple places, somehow confusing the installer. The answer is to umount it... then manually mount it below as shown.
Note: installing libgtk1.2 is just to have a gui installer instead of console/ascii looking installer
it worked for me.