After launching the CBS app, select "Already a subscriber? Sign up." Next:
  1. You'll be presented with an activation code.
  2. Visit cbs.com/appletv on your computer or mobile device, enter your activation code, and select "Submit". 
  3. You'll be taken to the Sign In screen, and under "I have a CBS Account", you'll see social media icons. Choose the icon that corresponds to the social media account you used to sign up for CBS All Access. 
  4. After signing in, you'll see a confirmation screen, your Apple TV screen will refresh, and you can start watching CBS All Access!