Help installation FlightGear-2018.3.5.dn on OS X 10.10.5

[stecca]

Hi at all, Ciao a tutti,

my name is Stefano from Italy,
I'd like to install FlightGear on my OS X 10.10.5 Yosemite. Download was ok, but mounting disk image appears an alert: (translated) "FlightGear-2018.3.5.dn no file system available".
As I red, searching for other topics, I try to install in an external disk also, but anything run.

There're other ways?

Thank you at all