If you are still on an older version of iOS, namely 3.1.3, you will no longer be able to download Apps from the AppStore. For those with first generation devices that no longer support iOS updates beyond 3.1.3, they can download apps to the iTunes on their PC/Mac and transfer from there. A direct download to device is no longer possible. So its time to stop being lazy and get  that update going. Seriously, you are missing out on quite a lot on iOS 5, iMessage?