Hi there.
I’m using the mac version of Zbrush 4 and for the life of me I can’t make it show up on the zplugins menu.
Here’s my folder structure

It’s in the wrong place. Downloaded plugins should be installed in the ZStartup/ZPlugs folder.
See here for general instructions:
http://www.pixologic.com/docs/index.php/Plugin_Installation
Yep that was it!
Thanks man!