Forum threads tend to get confusing after a while.
The simpler answer is that you can turn it on but it's considered less than completely safe. Or not as safe as it should be. In the current released software (15.09) if you turn it on it will try to stabilize always, even sitting on the ground or after a crash.
In the next release that's been fixed by allowing a switch to control it so you can arm, take off, and then turn on Always Stabilize When Armed using the switch.
Knowing the safety concerns you can use it if you want. It's enabled by going to the System tab, settings, FlightModeSettings, AlwaysStabilizeWhenArmed and setting it to true. Then click on the two icons near the top of the window with arrows that point up. Next go to Configuration tab, output sub-tab, and check "motors spin at neutral output when armed and throttle below zero". Click save in the lower right. The next time you use the board always stabilized will be on.
In the next release it will be much simpler to turn on and safer because it's controlled by a switch.