It's on the watch. You have to go through the settings page to give it an authentication token to allow it to communicate with your account. That superficially appears to work (I haven't checked the logs or anything), but you get that error message when you try to enter the app on the watch afterwards.

Thanks - it will be totally awesome if you can get this to work. I'll never run out of coffee again

BTW - the app is called 'Trebble' on the watch, but in the watch app store you'll find it under 'Checklists for Trello'.