If you want to use the cheat codes, you have to enter the cheat menu which you can find at the options screen. After you have done that, you will be able to turn the cheats on and off from the cheat menu then.
Disable damage: 6WL
Automatic Sat Nav: USETHEFORCE
Master code: CHEATINGMUTHATRUCKER
Infinite time: PUBLICTRANSPORT
Unlock Fast truck: GINGERBEER
Get $10 million: LOTSAMONEY
Unlock Diplomatic immunity: VICTORS
Level select: LAZYPLAYER
Unlock Bonus levels: JINGLEBELLS
Evil truck: VARLEY
Small pedestrians: DAISHI