If your PC-Doctor 6 is Dongle locked, then it requires it's Dongle to unlock it. It sounds like you are trying to use the PC-Doctor 5 Dongle with your PC-Doctor 6, which won't work. PC-Doctor 6 should have its own dongle lock, which should also have a copy of PC-Doctor 6 for DOS on it, so you won't be required to have a separate disc.
Hope that helps.