As stated, TMN (and TMNF, for that matter) doesn't have nor require a cd key... For coppers, p2p content (custom cars), and environments other than stadium, you need to purchase TMU(F); either boxed from a store or etailer, or downloaded from TM's website or from Steam.
For what it's worth... You can get a boxed copy of TMU from [To view links
please register] for $11.90 plus shipping. I'm not too sure on the specifics, but I'm sure you either use that key to authenticate your TMNF directly, or you install TMU from the disc and install the 'Forever' patch.