What is this? How do you enter?
You need to have PokerStars downloaded. When you log on to the lobby, launch the Poker School link on the top menu bar.
You can use the League link on the top menu and then take the core course and quiz. When you pass it, you'll automatically be eligible to play in the Pokerstars PSO games (they are found under Tourney/Private in the game software).
I've played some and have finished in the money a few times. I'm still mostly unfamiliar with the leaderboards and how to get added money from finishing well over multiple tournaments.
Hope this helps.