It might be obvious for those that have setup their own site controller, but I bought my APs through broadbandbuyer.com in the UK and they host the cloud controller themselves and supply pre provisioned, with themselves as super admin, and myself as admin.
It took me a lot of searching, and frustrated attempts to log on before I found that the relevant password for the plugin is for the Site, as seen in the Settings > Site > Device Authentication at the bottom. Clicking on the dots shows the password.
Hopefully this helps save others some headaches...
On a related setup issue as a new user. I had errors starting straight onto the latest version. Eventually I got a version 1.xx plugin working and then upgraded through a series of them all without issue until I got to the latest. Not sure what was the problem, but all working now.