To use liveries in game:
Choose a car in game(remember the number on the car). Click tuning/customize, and "create dir".
Find the created folder in: Steam\steamapps\common\rFactor 2\UserData\player\Settings\Howston_DISS_1974\(number on car) Unpack zip-file. Paste all files there, and choose it on tuning/customize in game. Voila!