Make sure you update your game first, and always launch the game through 'LaunchIV.exe' or it won't work with mods
To install cars you'll need either SparkIV or OpenIV, I'd suggest starting with SparkIV
Fire it up once you've downloaded (and installed) it and hit the big 'browse' button, then go to pc/models/cdimages. Now it depends on what kind of files the car pack had..
if it's one file (a vehicles.img) just hit import and select that file (it'll be highlighted in blue once you've replaced it) then hit save and you're done.
If it's however a folder full of files (.wtd and .wft files probably) open up the vehicles.img first (in SparkIV) then once it's open just do the same as I said before (import, select all the files, then hit save).