Have you tried running the setup.exe directly from the DVD yet? They added that to the Installing and Playing the Game part of the guide (a whole 3 lines of "help" on the 2nd page of the manual).
I can't say I've heard that complaint yet, but there's a lot of threads here to try and get through. I do know some people said Steam wouldn't read the disk, instead forcing them to download the game. I don't know if that's the same as your issue or not, though.