The app isn't something you need to download. You need to activate the find feature on the device's setup. The app is only so you can use your iPod to find some other device which has been lost.
Find my iPod Touch (iOS 4.2.1 and greater) - http://images.apple.com/ipodtouch/built-in-apps/find-my-ipodtouch.html - has to be enabled before device is lost. It also requires the device be on and connected to registered WiFi network, and not having been restored by the finder (if it was really stolen then the thief has probably restored it). If you have activated the "Find My" feature, go to https://www.icloud.com/