You shouldn’t normally need to find the plugins folder these days. Most plugins can be installed directly via the Extension Warehouse, The Sketchucation Plugin Store or by using the Install Extension option in Preferences.
It is not recommended to install old plugins, get new versions because most have been updated to work with the new Ruby system.
Having said that you can try this to find your folder.
Mac OS X - the default location is:
Version 2013 and newer: Open a new Finder window, press and hold the Option key on your keyboard, the click Go in the menu bar > Library > Application Support > SketchUp # > SketchUp > Plugins