Acorn TV is available on fourth-generation (tvOS models) Apple TVs. Search for Acorn TV in the App Store, then select the channel to add it. If you are already an Acorn TV subscriber, you can use your regular email address and password to log in. If you are a new customer, you can sign up for a free trial or purchase a membership through iTunes.
If you've previously signed up through iTunes, you can use the "Restore Purchase to Sign In" option on the app's home screen in order to sign in.