In the Squeezebox Server interface (link to the Squeezebox Server page accessible through the Max2Play Interface) you can find the Settings page on the bottom left of the interface page. There, you need to go to Plugins -> 3rd party plugins -> Spotify by triode. Make tick at this option and disable the official Spotify plugin. After that, just restart the server and log into your Spotify account in the Settings under Advanced -> Spotify.
How do I activate Spotify on the Squeezebox Server?