Here's how to unhide invisible directories under OS X (otherwise people will not see the ".blender" folder):
Open Terminal and type
defaults write com.apple.finder AppleShowAllFiles true (+ hit enter) killall Finder (+ hit enter)
To hide them again:
defaults write com.apple.finder AppleShowAllFiles false (+ hit enter) killall Finder (+ hit enter)
The latter is important … OS X is basically Unix, so when everything is unhidden so the user can trash it if they don't pay attention, things can go very wrong =).
I don't know if there is perhaps a more userland way of installing plugins for blender .. it seems like there should be, since poking around in the innards of apps and opening invisible directories is not something the user should normally do. I bet there is a shiny button somewhere in blender and I just don't know it.