I'm sorry to see that no one has offered any help here to you. I too have this same issue. I received my iMac as a replacement for one that had Apple Care coverage. I neither have a purchase receipt of it and only the work order stating it was given for unrepairable iMac. The local store neither set this iMac up for me or gave any assistance as to how to do it. Being a new computer and Mac user I chose to use my old Apple ID. When thing went bad, I too received this 'purchase ID' prompt. Which of course does not exist for this iMac. I stupidly registered it with the only Apple ID I new to be active for any App's bought and of course, Mt. Lion is NOT one of them!
If you've gotten a solution. I'd appreciate your advise. I find it reprehensible that Apple cannot tell that device came preinstalled with ALL applications when purchased as part of its STANDARD operating system!