mac

Open your ~/.bash_profile add this line

export NODE_PATH=/usr/local/lib/node_modules

Save ~/.bash_profile

1. Install Homebrew with command:
   ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
2. Use Homebrew install drush with commands
   brew tap homebrew/dupes
brew tap homebrew/versions
brew tap homebrew/php
brew install php55
brew install composer
brew install drush
 

Just moving to MAC OS and have problem with fn key when using Windows keyboard, fortunately there is a free application called Karabiner help you can swap fn and command key.