Install the Booster pack with vlted 4.6
Then Drag and drop the cars folder into your game directory, if done correctly then a box will appear asking to merge the folders, press yes!
ED is the tool to allow addon cars to work, you have to use it otherwise it wont work! Drop the Config, Resources and ED.EXE into the game directory.
Then open ed and locate NFSUNDERCOVER Directory it should be something like: Local Disk C/Programs files x86/ Need For Speed Undercover ect.
Once youve located the game, open it with ed and then it will load the config folder, simply press apply and it will install them.