does this apply to MAC versions? can I use this for .rb and .rbz plugins?

I am trying to install plugins for my sketchup pro. I went under my applications/sketchup and found no plugin folder that should be "preexisting" so I may drop the files into it. I found on the web that I have to go under preference>extensions> browse and install, but its for .rbz

Can anyone help me, and if so please explain at a clear level of understanding.

THANKS!