To find your Plugins folder do the following...
Open the Ruby Console from Extensions > Developer...
In its bottom a pane type or copy/paste this line of code.
Sketchup.find_support_file('Plugins')
It should return the path to the folder...
Use that to find/navigate to your Plugins folder...
If you have the SketchUcation tools installed there is a submenu item...
Extensions > SketchUcation > Open Plugins Folder...
Clicking on this opens your Plugins folder - so it's less of a hassle...
