The fileformat that iMovie uses on your iPad is a .iMovieMobile file, which your computer can't read. What you need to do is 'export' your file, which will save it to your camera roll. Then you can use 'explore' or 'finder' on your computer to find the file and transfer it over. There's a more detailed explanation here: http://www.tcgeeks.com/how-to-get-an-ipad-2-imovie-project-to-your-pc-or-mac/