Congrats on the new Prophet-6! Sorry to hear the mod wheel is loose. There are a couple of screws that hold the wheel hanger bracket in place, my guess is these need to be tightened and there's nothing physically wrong with the mod wheel itself. Please contact our dedicated support channel: support (at) davesmithinstruments.com
It's easy to open the lid with a screwdriver, and we have detailed instructions for how to tighten it up.