After it's imported into iPhoto there should be a small movie camera at the bottom of the thumbnail. Pictures do not have the camera. Double click the thumb of the movie and Quicktime should open and play the video.
Double click on the video. That should prompt Quick Time. If that doesn't work, two finger tap (Right mouse click) the video, Open with QT. Or open QT and try to open the video. Should work with double tap. Strange. I know this sounds stupid to say, but did you restart your mac?