git-flow completion requires git-completion to work. How exactly you go about installing git-completion varies wildly from system to system, so it's hard to give exact installation instructions.
By far the easiest way to install both Git and git-completion is via Homebrew, so you should pick that one.
brew install git bash-completion
Add bash-completion to your
if [ -f `brew --prefix`/etc/bash_completion ]; then . `brew --prefix`/etc/bash_completion fi
If you're using MacPorts, then you will need to install git-core with the
sudo port install git-core +bash_completion
And add the following to your
if [ -f /opt/local/etc/bash_completion ]; then . /opt/local/etc/bash_completion fi
sudo apt-get install git bash-completion
Add a .bashrc file to your home directory with the following line (or add this line to existing .bashrc file):