Go into the options screen, then go into the cheats screen and type the code in with the onscreen keyboard:

Effect - Code

Unlocks a H1 - Kubmir
It unlocks the Lexus GS 430 and the Cadillac CTS-V. - rimbuk
All Cities In Arcade Mode - roadtrip
No Damage - ontheroad
Faster Pedestrians - urbansprawl
Increase Car Mass In Arcade Mode - hyperagro
Argo Special Move - dfens
Zone Special Move - allin
Roar Special Move - Rjnr
Chrome Head - haveyouseenthisboy
Flaming Head - trythisathome
Yellow Smile Head - getheadj
Pumpkin Head - getheadk
Bunny Head - getheadl
Snowman Head - getheadm