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.
Install Git and bash-completion: brew install git bash-completion (Note: If this install fails with a 404 error, and you already have git installed, just remove the git part of this brew install) Add bash-completion to your .bash_profile:
if [ -f `brew --prefix`/etc/bash_completion ]; then
. `brew --prefix`/etc/bash_completion
fi
Snippet from https://github.com/bobthecow/git-flow-completion/wiki/Install-Bash-git-completion