First of all you need the latsest firmware. if there is a device option then either you don't have the latest spotify of chromecast firmware or you are not on the same network.
Google just started to roll out firmware 1.16.44433 but still Spotify is not working. you see the chromecast device but when you connect you get connected but nothing happens music continues to play on the phone.
So the older version is still not working!
CC2 it should work. but only with the latest version of spotify on your phone or tablet.