HOT WHEELS UNLEASHED 2 - Turbocharged

Go to your start menu, press and select Unleashed Missions. The third tab in the missions is called Specialist, this is where you can check what missions will reward you with secret cars, they will have a car icon shown as a reward.

The easiest is the mission "Once Upon a Time - Complete the HotWheels Creature Rampage (Career Mode)". You need to win against all five bosses of the game and then come back here and collect the reward car unlocking the achievement.

There are other missions that you could get the car with such as "Get All Common Vehicles in the Game" in the Collector tab of the Unleashed Missions, but finishing career is surely the easiest and quickest way to get your first secret car.