the value was partmgr
So, for a while I thought this is not going to work - but then I read another post at Microsoft Suport forum which pointed out the "case cap/small" differenece.
So, I changed it from partmgr to --> PartMgr
And then OK - exit Registry then reboot and run FSX, it asked about the activation and all is working.
Thanks for your help. I wasted a couple of days and uninstalled, reinstalled several time before this fix.
Thanks again.