I've got it working now too (but am keeping my fingers crossed because I got it to work a couple of times yesterday and the day before, but it didn't "stick").
One thing that I did this time was, instead of clicking on Roku Channel and then from inside there, clicking on Newsmax (which is what I usually do), I didn't click on Roku Channel and instead, clicked on "Newsmax" on that Roku home screen. When I did that, it did come up with a login screen (asked for my email address and password). I entered my Newsmax Plus login there. That did bring up the Newsmax Plus screen. I had to hit the "Play" button to start the streaming.
Even when you're presumably watching Newsmax Plus, though, it doesn't say "Newsmax Plus" or "Newsmax +" in the bottom left on the screen of the show you're watching: it still says "Newsmax." But I guess it is still Newsmax, because Newsmax2 is now what you can get for free; and they do show a "2" when you're watching Newsmax2. But then what's the "Plus" for? It's like they have two names for the new subscription service, Newsmax Plus and also, just Newsmax.