Because "Z3D" can refer to different tools depending on the context (ZQuad, ZModeler exports, or custom bridges), it is important to download from the right source:
Given the ambiguity, I'll provide a general guide on how to approach finding and installing plugins for Blender, which might help you with your Z3D needs.
She clicked it.
Most people scrolled past. Elara, a senior environment artist burned out on photorealism, downloaded it on a whim.