PSE plugins - where are they stored?

I am having some issues with plugins.  First it would be helpful to know where the heck they are stored.  I just installed a new program and wanted the plugin for it to be in PSE (latest version for Mac) - I went through the process from within that software and it says it installed.  However, when I try to access it in PSE, I cannot see it (I can see the previous version and also a few plugins for programs I no longer use).  It would be very helpful for me to look at the folder in which the plugins are stored.   I wish I could include a couple of screenshots so you could see what I'm talking about.

and yes, I completely deleted the program and reinstalled it, rebooted the computer etc, 3 times.

thanks for any help anyone can give me.