If you're using version 1.11 of the CBS app, the ability to sign in directly from your device is currently not supported. But it's still easy to log in with these steps:  
  1. From your Roku device, launch the CBS All Access Channel. You'll be directed to sign in
  2. Select "Sign In".
  3. You'll be prompted to log in, then enter the activation code.
  4. Your screen will refresh, and you can begin watching shows!