Not Available yet on the NowTV Boxes or Stick, but with your PS4 under the NowTV App the Bonus stream games should be towards the bottom of the NowTV App Home Screen Page.
Don't own a PS4, but that's where i can find the bonus streams on my Youview box NowTV App.
Also to access the bonus streams on your PS4 if you can find them you need a full blown NowTV Month Sports Pass.
I found that if I watched a Sky Sports On-Demand programme (or started to watch) it would appear on my recently played on my stick and would play from my stick on my TV. Bit annoying to have to do it that way, but a fix until it becomes available across devices.
Can you see the Bonus Stream Games towards the bottom of the NowTV App screen home page on your Youview Box ?
What model Youview Box do you have ?
If you can see the Bonus Streams, then they only work with a ful NowTV Month Sports Pass (the day, week or mobile sports pass doesn't work apparently with Bonus Streams).
Looks like that model is the mini zapper Youview box from BT.
To be honest i am not familiar with this particular model and not sure if Bonus Streams are available on the NowTV App or not (perhaps somebody who owns this particular model can confirm either way).