I set up Home Assistant largely so I could try out this plugin, and I'm finding I need a bit more hand-holding than the documentation here supplies, perhaps just because I don't really know my way around Home Assistant yet.
Access token. It took a good bit of searching around to find that I could generate that on my profile page (didn't even know I had a profile page).
Home Assistant Discovery prefix: This should be... "homeassistant"? There's no UI for it that I've found in Home Assistant so far, just a mention of the default here: https://www.home-assistant.io/docs/mqtt/discovery/
Event Name Prefix: ???