As of version 2.0 (8th January 2004), JZKit contains a ZOOM interface, so these days you no longer need to download a separate ZOOM layer.
Prior to JZKit 2.0, the JZKit implementation of ZOOM's Java Binding was maintained by Mike Taylor <mike@z3950.org>.
If you are interested in its history, or if for some reason you really, really ZOOM-Java code that interfaces to JZKit 1.x, then you're welcome to download the old code, but please be aware that this obsolete code is completely unsupported.
I've made some notes on frequently asked questions that arise when running this software (and JZKit in general).