FSDS Mirror functions don't work!

I've just been modelling a bit in FSDS, when I remember that I've often tried to use the three mirror buttons but it never mirrored.
I would like to use it for the fuselage I'm just working on, but in which mode and how can I mirror?
I would like to use it for the fuselage I'm just working on, but in which mode and how can I mirror?