Please read this general plugin install faq...

In general, on MAC, for R15 it is best to install any plugin to the user/plugins folder.

on a PC, or pre R15 MAC,  you can install to user/plugins or APP/plugins , it does not really matter which.

WireShader Pro+ is a shader plugin, so you might want to read this, if you are not sure where to find the plugin after install.