This is for units manufactured after September 2015. Apparently you can't get your ISN via the app with this Android APK (you can with iOS version). I've discovered that your "ISN" (in order to get your "auth code" from King Song) is your Bluetooth MAC address (6 groups of two hexadecimal digits) but "reversed". For example "12 34 56 78 90 AB" becomes "AB 90 78 56 34 12".
EDIT: Note that your ISN is not your auth code. It's what you need to give King Song in order to get your auth code.
EDIT2: It has come to my attention that with the newer firmware (I had an older version) you *can* get the ISN via the app.