You can access SBS On Demand in one of two ways on your Sony Android TV.
1. Sony Select row on the Home screen of your device.
This link will take you to the the Play store listing for the Android TV app where you can download it to your device. Once you've installed the app, you'll be prompted to sign in or create an SBS account
2. Play store
Go to the Play store on your Android TV.
You'll need a google account/ID to download the app
Go here if you haven't got one yet: https://accounts.google.com/SignUp
Search for SBS On Demand
Install the SBS On Demand app. Voila!