You're not trying to run this on your Windows system, are you? If so, you can't. Apps purchased from the iTunes Store are for iOS devices only (iPhone, iPad, iPod touch). If you want an app for your Windows system, you'll need to buy that app elsewhere.
If you are trying to open it on an iOS device, give us details about the device and what happens when you try to run the app and we'll go from there.
Regards.