I don’t use HASS.io so I can’t be much help I’m afraid! The repo you linked is for addons which are additional Docker containers, rather than for components. I think you need to enable the SSH or Samba addon which will then let you modify the config (and add custom components)?

In other news, the switches now work! Turned my heater on via Alexa yesterday, as a smart home device instead of having to address the skill directly.