How can I watch iTunes purchased movie on my Mac if it is missing in my library? I have purchased Cars 2 HD movie on my iTunes. I can't find it and/or play it on my Mac now. the only option is to buy it once more. I have just one iTunes account. when go to the movie bar in my library, there's nothing there, just the intro screen. Please help.
This Apple support article deals specifically with downloading past purchased content. You want to check the section labeled Mac or PC since you're trying to download purchased content in iTunes on a computer.

From the article and assuming you're using iTunes 11 or later:


Open iTunes.
If you're not already signed in, choose Store > Sign In and type your Apple ID and password.
If you're in the iTunes Store, click the Library button, in the upper-right corner, to go to your library.
Make sure that you have "Show iTunes in the Cloud Purchases" enabled in your iTunes preferences by going to iTunes > Preferences in the Store tab and clicking OK.
In your iTunes library, click the content type you wish to download using the content selector in the upper-left. Choose Music to download music, Movies to download movies, and so on.
Locate the item or title you wish to download.
Click the download icon to download that item.


And as 2DD8847 points out their answer: being able to do this is country dependent. You can only do this where an X appears in the media category for the iTunes in the Cloud availability by country table found in this support article. And note that for some countries, under the Movies category, there's a caveat that "Some previously purchased movies may not be available in iTunes in the Cloud."