First you have to install a CDROM-filesystem. Os3.1 comes with a CD-filesystem, but it's pretty crappy. Os3.5 and OS3.9 both come with CacheCDFS. You can also look on
aminet for a decent filesystem. I believe idefix comes with one also.
Now you have to check your storage/dosdrivers drawer if it contains a CD mountlist, something like CD0.
Start a texteditor and edit this mountlist.
Change the device to the appropriate scsi device and change the unit to the appropriate unit number.
Save your changes and move the CD mountlist to DEVS:dosdrivers. The mountlist will then be automatically mounted when you startup your system.