It looks like there might not be a Roku app just yet, but there will be one in the near future (see link below).
https://forums.att.com/t5/Streaming-to-Devices/Another-way-to-enjoy-DIRECTV-NOW-on-Roku/td-p/5101580
The above link also states, "Did you know you can stream from 20+ network apps using your DIRECTV NOW credentials on your Roku? DIRECTV NOW subscribers can enjoy some great on-demand programs and movies."
The list can be found here: https://help.directvnow.com/hc/en-us/articles/215395423
I hope this helps!