git: 'flow' is not a git command. See 'git --help'

Remove git-flow and install it manually.

On Linux:

sudo apt-get remove git-flow && wget --no-check-certificate -q -O - https://raw.github.com/nvie/gitflow/develop/contrib/gitflow-installer.sh | sudo bash

On Windows

If you work with cygwin make sure util-linux and wget are installed. Then run this with cygwin:

wget -q -O - --no-check-certificate github.com/nvie/gitflow/raw/develop/contrib/gitflow-installer.sh | bash

For further information check the project page on github

As mentioned in the comments, most likely /usr/local/bin isn't set in $PATH. Check it by doing echo $PATH, and if it doesn't show /usr/local/bin, add it to $PATH.