You need to log into the Spotify website to change these settings.
After logging into it, go to Profile directly (which is also under Accounts). Once you're at the Edit Profile page, you should see at the very top:
Use Spotify on your devices
Set your device password to use Spotify on your Sonos, Squeezebox and a whole heap of other devices.
Then just set your password for your devices. It would be an numeric username, but it links to your Spotify account nevertheless. I just used that to log into my WP8 Spotify account and it plays and syncs perfectly.
Yeah I can't see the passwords for devices setting via web browser either. It's incredibly frustrating, I've been trying to download some songs to my Lumia 920 for offline use but I don't seem to be making any progress.
Thanks for the thread. This definitely needs to be fixed in the app. If nothing else, a detailed error message needs to be displayed.
I think that setting up a Spotify (not Facebook) userid fixed the problem on the server for me. However, the app had no obvious way how to change the user id / password, so I had to uninstall/reinstall. Fixed it!