The easiest way to get NIX is via homebrew. G-Node homebrew formulae are maintained in the G-Node tap. Install nix, including missing dependencies, with:
# tap the repository
brew tap g-node/pkg
# install with
brew install g-node/pkg/nixio
# If you want the latest development version:
brew install --HEAD g-node/pkg/nixio
# to upgrade an existing installation to the latest release
# replace 'install', with 'reinstall'