Err. How do you install plugins in OS X anyway? There is no app/plugins folder that I can find in application support or the apps package..
Well, i have never used OSX, but if you want to install a plugin manually you should have a folder structure similar to ~/Library/Application Support/Superpowers/systems/game/plugins and inside that create a folder structure like this name_of_author/name_of_plugin
Or if you want to use the official way, go to settings tab in superpowers app and will appear the list of plugin available for each system.