Absolutely impossible to draw here, but the two ferrocenyl rings face in opposite directions, and so therefore do the chlorine substituents. You have a C2 axis that bisects the iron and is perpendicular to the plane that contains to two chlorine atoms. That plane is also your mirror plane. There's also an inversion center. If you can see how the inversion center switches the places of the two chlorine atoms, then you should be able to see the mirror plane.