why do movies i purchased not show in iTunes on my mac, but show on my iPad

The movies should copy to your computer when you sync your iPad with iTunes assuming that the computer is authorized to the iTunes Store account through which you purchased the movies. Have you tried that, and if so do you not then see the movies in the Movies area in iTunes?

That's the message you'll get when you try to connect to a different iTunes library. Did you perhaps change computers, or make some sort of change to that computer, such as using a different user account, or redo iTunes in some way?

This was happening to me, as well. I opened iTunes and went to my Purchased directory and noticed that the movie that would not sync was unchecked. (Which has never been a problem before) But once I checked the box, the movie was available to download and the download worked. I hope this helps.