I am trying to use this particular piano plugin on my GarageBand DAW on a Macbook.
http://www.supremepiano.com/index.html?piano1.html
I downloaded it, unarchived (unzipped) it and installed the .pkg file. 
Now I understand, that if the install worked properly, on my Macintosh HD, under Library --> Plugins, I should see this Plugin, but I dont.

I also tried opening GB, creating a track(software instrument), open Smart Controls and click on Plugins. I don't see the plugin I downloaded, only the default Apple Plugins.