Game cards are unlocked by tapping the Wumbozi machine in the Krusty Krab and completing the score total that is shown as SpongeBob and Patrick are warping into the game.

As soon as you get a new card, that card appears in the end game screen where the score appears. That new game will be added to the games that you play by tapping the Wumbozi machine (it's the first game you play in a new Wumbozi Game after collecting a new game card).  The new game will also appear in the collection of games in the Playlist and in the Collection Book.

You can tap Spongebob in the Krusty Krab main menu to enter the Collection Book menu and see which games are available to play. The cards with a lock are the ones you have not yet unlocked in the game. The cards with a box are the ones available for purchase.