Earlier models of the Iomega Zip drive used an external parallel port connection, as opposed to the later drives' USB and FireWire connections. You can convert the parallel port to an USB connection using a parallel-to-USB adaptor. Several companies make this type of adaptor, but all work in a similar fashion. Put the adaptor on the Iomega Zip drive, then connect it to the USB port.
Connect the parallel end of the parallel-to-USB adaptor to your Iomega Zip drive.
Plug the Iomega Zip drive into power.
Connect the other end of the parallel-to-USB adaptor in to your computer's USB port. Wait for the computer to recognise the drive as a mass storage device before attempting to access it by going to "Start" and "Computer."