Regardless of which instructions you received, if your lines are ready to be used for DIGITs, you could download the app from either the Apple store or the Google Play store and use it. Since it isn't working, there are a few things to check. First, when you registered, did you sign in with either the Primary Account Holder line or the T-Mobile ID tied to the #(s) you are trying to use? If so, please visit https://mydigits.t-mobile.com/consumer and login with your T-Mobile ID. There are two sections that should be present - lines you can use and lines you can manage. If they are no lines in the section for you to use and you cannot move them, you may need to resubmit your request.
You could also contact our Beta team to see if there are any other issues that may have prevented your lines from being allowed. You can contact the Beta team at 1-844-889-5651 from 3:00 a.m. to 10:00 p.m. PST.
Hope that helps so that we can get you a resolution quickly.