No idea for Mac. Officially I don't give support for macOS, because I can't test it, but if you figure out how to get this working, then the plugin should start working, too.
That's basically the problem on Mac. Each installed does not get fully registered to the users environment. Instead, you have bundles of software, which are running independently.
I figured it out my self
This is my ending script in Simplify3D:
G28 X0 ; home the X-axis
M84 ; disable motors
M140 S0 ; turn off bed
M106 S255 ; set fan to full speed
M109 S50 T0 ; wait until heater is 50°
M104 S0 ; turn off heaters
M107 ; turn off fan
Now after printing is done it sets the fans at full power until hotend reaches (cools down to) 50° then shuts of heater and fans.
I pretty sure my printer wont be ubducted by aliens if the fans are left on after printing is done, but I don't like things in general to be in motion or in use more than necessary 🙂