Here is a link to a YouTube video showing the NXT Whac-A-Mole in action!
Summary:
When the score indicator reaches the top of the tower, an Ultrasonic Sensor tells the master NXT brick to lower the score indicator and begin a new, faster, harder game for the player.
Using Bluetooth, the "Slave" NXT brick tells the "Master" brick whenever a mole is hit. The moles are made out of many engine chassis from such models as the Technic Bulldozer. When hit, the engine chassis pushes down onto a touch sensor, which tells a NXT brick to add a point to the player's score, raising a point indicator attached to a score tower.