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 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!