Go to the 'Main' menu (where you can select Free Drive, Trophy etc.) and press 'Shift' to bring up a box that will allow you to enter cheats. Enter the following case sensitive codes (without the quotes) to enable the corresponding effect.
Unlock ALL vehicles: Enter 'ALLCARS'
Unlock ALL Cars: Enter 'ALLCARS'
Unlock ALL Trophies Access: Enter 'ALLTROPHIES'
Unlock ALL Tires: Enter 'ALLTIRES'
Unlock ALL Frondt and Rear Differentials: Enter 'ALLRDIFFERS'