We are not Apple. We are simply a collection of Apple users offering advice. Contact the Apple retail store where the Genius Bar employee gave you the wrong information. Your system is not compatible with windows 8. Contact Apple App store support regarding Mountain Lion return with the link provided above. Apple has excellent customer service, generally. Talk to both and explain the situation. They are generally very reasonable, especially when it comes to any error on their end.
It will not work. Again:
Which Macs support Windows 8?
For more information, see this article.