Are you sure thats how it works? The Apple website just says
Download an app and you’ll automatically get the chance to win a $10,000 iTunes Gift Card, an iPod touch, a Time Capsule, and a MacBook Pro. Just go to the iTunes Store, browse the App Store, and download your best app yet.
Im not sure, but i think they have to be apps that you haven't already downloaded. It shouldnt be that hard, theres almost a billion apps and im sure there are plenty of free apps to download. im just downloading whatever i can get for free. even if i don't plan on using them