"Installer" is a standard program used to run installation scripts. It is contained on the Install3.1 disk but unfortunately not copied to HDD during OS installation. So copy it manually to Sys:C and to Sys:Utilities (different scripts expect Installer to be in one or the other location, so copy it to both), then the IDEfix installer will run.
There is a new version of Installer on Aminet which is to prefer:
http://www.aminet.net/package.php?package=util/misc/Installer-43_3.lhaAgain, copy the Installer program to both Sys:C and Sys:Utilities.
When the installer lets you select your CD-ROM drive, use scsi.device as driver, not atapi.device. Then you have less hassle when removing IDEfix later.
Bye,
Thomas