i rented a movie less than a month ago and it does not appear in iTunes
I rented a movie in iTunes, it was less than a month ago, but it is no longer in iTunes. I have looked under movies, purchased and unwatched.
MacBook Pro, Mac OS X (10.7.5)
You rented the film on that Mac, and did you start watching it (even if only for a few seconds) ? After starting to watch a rental they expire after 24 hours if you are in the US, 48 hours elsewhere. If you hadn't started watching it then contact iTunes Support : http://reportaproblem.apple.com