The Zip Plus has the ability to run from either a parallel port or a SCSI controller, and it costs a lot more than any other model of Zip 100 drive. In addition to its being more expensive, it has serious limitations as a SCSI device. Iomega seems to have removed all references to the Zip Plus from their website, but the problem is essentially that the Zip Plus may be unreliable (i.e. data may be corrupted) if it shares the SCSI chain with any other SCSI devices or if it is connected with any cables or adapters other than the cable which comes with it.
If you're considering one of these, you might be better off getting a SCSI Zip in addition to a Jaz Traveller (for when you want to connect it to a parallel port). There was a website which discussed using a SCSI Zip with a Jaz Traveller, but it seems to have vanished. However, I have a copy of it which I can e-mail to anyone who wants it.
If you already have a Zip Plus, then look through the section which deals with the way you've connected it: parallel port or SCSI. And don't forget about the drive's limitations if you've connected it to a SCSI controller.