The "Find my iPod/iPhone" app is actually what you use when you're tracking down a device. For example, if my iPhone was lost I would use the "Find my iPhone" app on someone's iPhone or through iCloud.com to locate my iPhone.
The actual part that allows you to track down your iPod is built into iOS in the form of a switch in the iCloud settings. [http://www.apple.com/icloud/setup/ios.html|This guide] walks you through activating it. Take a look at it and if it seems like you had already followed those steps then head to iCloud.com, sign in and go to "find my iPhone" to hopefully locate your device.
Good luck!