One: it isn't actually playing automatically. I'm willing to bet money that the guy in the video you linked has a midi keyboard, or is just playing with the typing keyboard. That "melody" sounds exactly like the basic dicking around I do on my keyboard when previewing sounds and samples. He's playing it himself, it isn't going automatically.
Two: I still don't see what you mean about having all of the notes in already. It looks to me like he's drawing them in himself. Never mind Nexus, I don't think there's a VST out there at all that automatically writes midi melodies straight into the piano roll.
Also, as MockOff said, if it's pirated and this is a software issue (which I seriously doubt is the case) there isn't anything we can do to help. If you've bought it, then I suggest taking a look at some tutorials for getting started in FL before you worry about genre specific stuff. I highly recommend the Beginner to Pro series. It's a bit dated, but relevant enough. That's how I got started.