You have to enter a racetrack (testing or race mode, offline or online doesn't matter) and below the box that says 'chat', there is a small dark line which is where you actually chat from. In that box, you can type these codes...
ISI_BABYFACTORY - you own all cars from that series and have 2 mil to boot in hard cash
ISI_SKIDMORE - gives you back a percentage of the money you currently own, must be typed multiple times if you are low on cash
ISI_SKENDERBEU - gives you a percentage of experience points that you currently have, again, must type multiple times if you don't have much experience