Update - I have finally managed to add my smart plug to Home Assistant Green. The difficulty I had was that all of the instructions implied that the Supervisor would be visible in the left panel - it isn’t. You can find it in Settings > Devices and Services. Once I had done that I looked in vain for an hour for a place to past the access token, and in the end decided to skip that step and press Start anyway. This worked and I can now see the smart plug on my dashboard.
Hopefully this will help other new users of Home Assistant Green.