And just to add to the answer already given, and add a bit of background..
What you've recently done is to convert what was a V6 into a TV360 box. 360 is Virgin's newest software platform which is completely different to the TiVo-based system you had before. 360 has some new features that the V6 didn't, some features are different, and some are missing. But you don't have a V6 anymore - and you'll notice I've moved your post into the 360-section of the forum.
That's where ozsat's advice comes in - the Pluto streaming app is one of the features that currently isn't available on the 360 platform, and until it is - you won't now be able to use it on your 360 box.
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