Followed the instructions, and it seems like you gotta use 3 fingers, and push to click.
Anyway, I'd highly recommend you try to use a keyboard binding to achieve the 3 finger click action. That should be easier to do because it is gonna be tricky with the trackpad.
Goes without saying, never be without a backup mouse when you need to do work in an app that uses the middle click but BTT does gives you wings, really might be fine with a keybind.