I think you got ripped off as per microsoft: "As of October 30, 2015, the Windows 8 and 8.1 Pro Pack (which upgraded Windows 8 Home or 8.1 Home to Windows 8 Pro or 8.1 Pro, with Media Center) and Media Pack (which added Media Center to Windows 8 Pro or 8.1 Pro) are no longer available for purchase from Microsoft. Previously purchased feature pack licenses will still work, and valid feature packs may still be available for purchase from third-party retailers. Support for these products is still available."
But, just in case you already have it "The Media Center upgrade installs automatically when you "ADD FEATURES" to Windows 8\8.1 Pro, and enter the key you were furnished when it was purchased when requested..."
so there you go, enable it by going to settings, apps, features and enabling it.