The easiest way to get NIX is via homebrew.
We are included in homebrew science as nixio.
Install nix, including missing dependencies, with:
brew install homebrew/science/nixio
# OR: tap into homebrew-science
brew tap homebrew/science
brew install nixio
# If you want the latest development version:
# NB: replace 'install', with 'reinstall' to upgrade
# an existing installation
brew install --HEAD nixio