brew update && brew install elixir
mix local.hex
mix archive.install https://github.com/phoenixframework/archives/raw/master/phoenix_new.ez
Install Postgresql (if not installed already):
brew install postgresql
createuser -s postgres # if not created already
Install Node.js (get a recent one if you have pre 5.0 node):
brew install node
# if you use NVM, you know what to do
Begin from here: http://www.phoenixframework.org/docs/up-and-running