With most of our games, there is no longer a way to manually enter the serial number or download the game directly. Instead, the game's licensing is applied upon installation by the MacGameStore client app. If you haven't installed the MacGameStore App, please download it from here: http://www.macgamestore.com/mac-app/
Once installed, double click on the MacGameStore app to launch it. Make certain you're logged in by looking at the top right - your should see your username. If not, click the button that says "Sign in or Sign up" and log in.
On the left hand side of the MacGameStore client window, there are icons running vertically. The top one is labeled "Store," and looks like a shopping bag. The one below that is "My Games" and looks like the play button from a tape player. Click the "My Games" icon. Just to the right of the icon, click on the word "Purchases."
This area is your purchased game library. You'll see square icons of each game on the left, with action button to the right. If a game's icon has color, that means it's installed and its action button will say "play." If a game's icon is gray (with a little cloud in the corner) that means the game needs to be installed and its action button will say "Install."
Clicking "Install" will start the game downloading, and the action button will also change to "Downloading." Then when the download is finished, the game will install. Most games will be ready to play at that point.
If there are no games listed, but you've already purchased them, go ahead and go to the game's info page in the client (the quickest way is by starting to type the game's name in the "Search" field at the top of the window). Once on the game's page, click the "Buy Now," button. The client will tell you, "you've already bought that" and you can click "Install now."
The game will automatically download and install, and then the game should be listed in your MY GAMES -> PURCHASES area. Then you can click "Play" to start your game.