You need to download the 30 day trial of PSP7 from the downloads here at FUO if you dont have it for painting skins. Then go to
www.freewebs.com/dtr2rookie for guides on how to get the skins in game.
Chassis are a lot harder and you need to download the de2 editor, also available here. That way you can open up the files that have the chassis models in them...then go back to that rookie site and it will tell you how to mess with the chassis even though the guides there are for making add on tracks...but use the same principles for making chassis. It's fairly hard to edit tracks and chassis though so don't give up easily.