Just doing a sanity check: You did remove the shell to make sure the receiver wasn't tucked in its holder? If its not there then I would just return it for a refund. I'm not sure if you can find it separately (maybe Ebay?) but I'm guessing it won't be easy to find and might be costly. For whatever reason people always lose the receivers for wireless devices but rarely do I see a receiver without the device.
Nothing under the shell. It missing. I hope asus support can help me out.