There are two ways to do this:
Add channel via your remote
 1. Go to the Roku home screen by clicking the "Home" button on your remote.
 2. Navigate to the Roku Channel Store by selecting "Streaming Channels" from the menu.
 3. Navigate to the "Movies & TV" category, then find and select the "CBS All Access" channel.
 4. Once on the CBS All Access screen, select "Add channel".
 Add channel via desktop/mobile device
 1. Access by clicking here.
 2. Next, follow the prompts to either "Sign in" or "Create a Roku" account, depending on whether you are a first-time or returning Roku user.