There are a few ways to do this, depending upon your Apple TV model. 

If you subscribed via Apple TV 3 or earlier:
  1. Launch CBS All Access on your Apple TV.
  2. Navigate to Settings > Link Subscription to CBS Account.
  3. After choosing "Link Subscription to CBS Account", you will be presented with an activation code, and prompted to visit cbs.com/appletv on your computer or mobile device to enter the activation code. 
  4. After entering your activation code, your Apple TV screen will refresh. 
  5. You will see a confirmation screen that reads "CBS Account Activated". 
  6. Now you can sign in to CBS All Access on any device!

If you subscribed via Apple TV 4 or 4K:
  1. Simply sign in using your CBS All Access email address and password.
That's it!