atapi.device alone won't suffice. Install the entire package on the Amiga. I just meant that when you're configuring your CD0 mountfile, use atapi.device and not scsi.device. There are other tools in that package you need, too. Fortunately, the installer should guide you through everything.
And make sure you have the file 'Installer' in C: or SYS:Utilities before you begin. Hopefully you do, since the Toaster instal script started up, but it's possible the Toaster disk had a local copy. If that's the case, just copy that one to one of the above places on the hard drive.