So here’s the answer to the original question.
It’s broken. It really used to work. Now you have to do the following.
- download and unzip the add-on
- place it in the
~/Library/Application\ Support/Local\ by\ Flywheel/addons
directory, or your equivalent - Restart Local
- Go to Installed Add-ons page and activate your add-on
- Restart Local
The new add-on should now be available.