Where and how did you initially create your account?
I used my PC but if you go to the NOW TV website on any device other than the TT box and sign in it will show your active passes. Once this is confirmed on there you shouldn't need to log in again. You can use up to 5 additional devices as well with up to 2 at the same time.
I have been a Now customer for years so I'm not sure about registering process on the box itself. The best way I have found is the NOW TV webpage from a mobile, tablet or PC. @ferguson is well versed on doing this so follow his guidance if you can't access the webpage.