i used to have a powerbook and zip, so i know it's a bit of a hassle. anyway, what you want to do is to get a SCSI adaptor to convert the HDI 30-pin SCSI on the powerbook to the 25-pin SCSI cable that came with the zip drive. i think you can get either a cable to do this (one end is 30-pin, the other 25-pin) or a port which has switchable termination. i'd go with the port version so you can use the powerbook in SCSI disk mode.
-r.