Unlock all cars: onecarefulowner
Unlock all stages and difficulty levels: hellocleveland
Reverse stages: rorrimskcart
To put in the cheats go to "Drivers" in the main menu. The click "create new driver profile". It will ask you to put in a tag (3-letter name). Put in whatever you want. Then it will ask you to enter your driver name. Input the cheat code there and it should work.
Let me know how you get on.