Use the following steps to add new cars to your TrackMania Version

1. Select one of the car brands from the menu by clicking on it.
2. Use the mouse to go to your favourite car and download it by just clicking it once and select the location on your computer to save the file to.
3. Do not unpack the file after downloading it (it is in zip format). Just move the file to the correct path of the game depending on your TrackMania version. Beneath you see the different paths where the cars should be placed.

          TrackMania Nations Forever and TrackMania United Forever

• All enviroments :*User documents folder*\TrackMania\Skins\Vehicles\CarCommon

          TrackMania Original

• Rally :TrackMania Original\GameData\Skins\Vehicles\Rally
• Desert :TrackMania Original\GameData\Skins\Vehicles\American
• Snow :TrackMania Original\GameData\Skins\Vehicles\Snowcar

TrackMania Sunrise Extreme

• Bay :TrackMania Sunrise\GameData\Skins\Vehicles\BayCar
• Coast :TrackMania Sunrise\GameData\Skins\Vehicles\CoastCar
• Island :TrackMania Sunrise\GameData\Skins\Vehicles\SportCar

TrackMania Nations

• Stadium :TrackMania Nations ESWC\GameData\Skins\Vehicles\StadiumCar

TrackMania United

• All enviroments :*User documents folder*\TrackMania United\Skins\Vehicles\CarCommon

I only tested the cars with TrackMania United, but most cars will probably work in the other versions as well.