I think you have to add DirecTV Now to your Roku channels, then log in to your DirecTV Now account, and you should be good to go.
Here's a ;ink to the instructions on DirecTV's site: https://help.directvnow.com/hc/en-us/articles/115000637383-How-do-I-set-up-DIRECTV-NOW-with-a-Roku-device-