If you've been trying to use MediaCenter.IsApplicationActive and wondering why it doesn't work then wonder no more.  It can't be made to work as it doesn't actually exist, it's documented in error.  To get the same functionality you can use MediaCenter.IsForegroundApplication instead like this:

window.external.MediaCenter.IsForegroundApplication(entrypointGuid)

Where entrypointGuid is the guid of the entrypoint for the current HTML application - this is what you'll have used when you registered the application.