I have run into this a few times, and it finally got me annoyed enough to write something about it. The Alpha/Linux installer will only see SCSI cdrom drives. I have tried to install on two boxes with IDE cdrom drives, and when I tell it that I want to do a CDROM install, it complains that /dev/scd0 can not be mounted. This is wrong. It should be mounting /dev/hda. If I didn't have a net connection and/or another SCSI cdrom, it would be a show stopper.
Where are you seeing this message?? What type of machine are you installing on (SRM/ARC)? What boot line are you feeding to the machine? We have a handful of alphas in the lab with IDE drives on them and have no problems performing a CDROM installation from these machines. Please reopen this bug if you are continuing to have problems.