I tried following the README by running netlify addons:create nimbella and netlify addons:auth nimbell.
netlify addons:create nimbella creates the addon, netlify addons:auth nimbella creates the auth link and opens the browser, but redirects to https://nimbella.com/.
I was able to use an existing account with nim auth export --non-expiring.
Feel free to close this if this flow is not intended to work yet.