If you've already got the infection, all you have to do is go to split-screen and choose a file (or make one if you've never made one before.) Then once you are in the split-screen lobby, the map should be of the 10th prestige infection. Go to options and make sure that PSN Unlocks is ON that is the most important part. Then, just start it right up

And once I start it do I have to press anything?

With a 10th prestige infection, no. It should just kick you after you start it and it loads up. If not, then once you are in the game, just end it.

Alright nothing happened so does that mean it's a scam or am I just not doing something right?

Could be either one.. If the map looked like the infection you got in a private match when you were in split screen, then it is most likely you messing up. You can try to push each of the d-pad buttons while in the game. But 99% of 10th prestige infections will just kick you right after starting the match in split screen.