how can i open movie on my mac air via usb?

Is the movie on a flash drive? if so, when you look at the movie file on the flash drive it should end with .mov or .mp4g or some thing like that, what dose it end in?

Or are you trying to use an apple usb super drive to play a dvd?

or do you ahve some sort of other USB device? such as a blueray player.

Try the free VLC player if your Mac won't play the video in Quicktime:

http://www.videolan.org/vlc/download-macosx.html

If you need help on general use of your mac, try:

http://www.apple.com/support/macbasics/