In Cocos2d, we can create transitions between scenes, in order to replace the current scene with the following one. You can think of transitions in games as transitions between slides in a presentation, so we can animate them.

We can create six types of animations thanks to the class methods available: transitionCrossFadeWithDuration, transitionFadeWithColor:duration, transitionFadeWithDuration, transitionMoveInWithDirection:duration, transitionPushWithDirection:duration, and transitionRevealWithDirection:duration.

You can play with these methods, but we are going to use transitionMoveInWithDirection for this game, so you need to replace the following lines in goToNextLevel:

[[CCDirector sharedDirector] pushScene:nextLevelScene];

Instead, use the following lines:

    // Create transition
    CCTransition *transition = [CCTransition transitionMoveInWithDirection:CCTransitionDirectionLeft duration:0.5f];
    // Push scene with transition
    [[CCDirector sharedDirector] pushScene:nextLevelScene...