Look for the update your iPod option and select it. This will bring you to the main window for your iPod. The first is to plug your iPod into your computer and open iTunes (if it doesn't open by default), and select your iPod when it shows up on the top right side of the iTunes screen. I don't know if your iPod can be updated to iOS 7, but it can be updated to iOS 6 if you are getting the notification. Good luck! Let me know if it worked for you. It did not work on a newer app I tried though. However, this worked fine for Spotify and a few other apps for me. I guess the developers just don't find it necessary. Keep in mind some of the newer apps out there now do not have compatible versions for the older iOS versions. Also, make sure your iTunes and iPod have the same Apple account. Since you already tried this, you may have to delete the app from your iTunes account first and then go through these steps. You can't let the app update on the iPod either or it wouldn't work. Just don't connect your device to your computer during any of this process. It should then give you the option for a compatible version to install. Once it is fully installed in iTunes, go to your iPod touch, go to the app icon, find the app you want, and 'buy' and it again. 'Buy' it through iTunes and let it install into iTunes. Try going into iTunes without your device plugged into your computer and finding the app. How to get an App compatible to older iOS versions.

Requires a newer version of itunes