IIRC, you need an extension to use a Zip100 drive with a mac, but don't quote me on it.

Here is a link with a download for the driver and tools needed Here

It also has helpful instructions on how to get the drive to connect.

Just don't quote me on it, I've only used a zip drive on a PowerBook G3 and a Power Mac G3, so I'm not sure if it could be the same problem or not.