I suspect he's trying to access stuff on frmMain from frmPopup. If he'd have searched this forum he'd have found this question is asked in various forms at least once a week.
My preferred solution is to alter the constructor of the secondary form so it takes a reference to the first form as a parameter, which can be stored for whenever you need to access it.
You could also set the tag of the secondary form to reference the first form after creating it, but that would mean you wouldn't be able to reference frmMain from the constructor.