You can install and use the plugin as follows:
First, follow the Bitbar Getting Started instructions to get BitBar running on your system. Note: make sure you create a new plugins directory when first installing, don't just use the Documents directory itself - try creating it in ~/Documents/bitbar_plugins.
Install using the provider Homebrew tap.
$ brew tap johnmccabe/openfaas-bitbar
$ brew install openfaas-bitbar
Install a symlink with the specified refresh interval to the plugin directory.
$ openfaas-bitbar install
Note: When installing for the fist time you will need to manually restart the BitBar App, or select Preferences/Refresh all from its dropdowns.
The openfaas-bitbar plugin looks for the presense of a ~/.openfaas/config.yml file, and if present it will use that to decide what gateways to connect to and use their configured auth. To ensure you can see/login to your gateway, make sure that you've run the faas-cli login command for each gateway you wish to see in the menubar.
brew upgrade openfaas-bitbar