Having been frustrated by a problem with Napoleon: TW that seems to only happen to me, I decided to fire Medieval 2 back up. The installer says the CD Key is on the back of the manual, but all I see is an ad for TW: Eras, some logos and urls, and an ESRB rating. I checked the inside covers of the manual (since that's a fairly common location) and didn't find it there either.
I threw away the box, so if the key is there, that would explain it. I always (as far as I recall) check boxes for keys before throwing them out, but I suppose I may have forgotten this time.
I seem to have gotten around the issue by following the instructions here: http://shoguntotalwar.yuku.com/topic/70921 "Problem with game Cd Key" (it let me continue installing at least). Mainly I'm posting to see if the key was on the box, or I'm blind, or what.