Unfortunately there's no really easy way to get new cars and tracks. This is probably the easiest way:
For cars:
http://vdrift.svn.sourceforge.net/viewvc...data/cars/
For tracks:
http://vdrift.svn.sourceforge.net/viewvc...ta/tracks/
At each of those links you'll see a list of folders where each folder is a car/track. If you see a car/track you're interested in, click on the folder to go inside it. Then, click the "Download GNU tarball" link at the bottom of the page. That will let you download an archived version of the folder. I'm not familiar with OSX but I bet it's probably got a built-in way to uncompress the .tar.gz archive. Then you just move the uncompressed folder to the vdrift/data/cars (or vdrift/data/tracks) folder. Simple, right?