When you click on one of the games, you’ll be taken to a page about that games, just like other iTunes pages about albums or movies. Here you’ll find user reviews, a preview (but not a playable preview), and the “buy game” button.
Click that button to buy the game. When you do this, you’ll see the standard iTunes Store purchase actions like the progress bar in the top of the window and the download happening at the Purchased Items playlist.
