These get wired into an appliance cord or extension cord. So the “plug” is universal. For the dry contact application that would be used here you would wire the plug end of an extension cord into the input to supply.it with power (or use a 3.3v power supply into the programming pins) then wire the bell wire for the garage door opener into the output end. In order for tnhis to work after its flashed, you will need to do some work to the PCB but it’s really easy if you can solder. It’s just adding a jumper wire. If you need better pictures or directions I can try to help you out.
As far as your Mac I’m not sure. I don’t see why it would be an issue but if you ask on the Sonoff thread someone may be able to answer that.
For the core setup I can help with that when you get there it’s simply
If
garage door button (simulated) is pushed
Then
Flash Sonoff switch (actual Sonoff device) on for 500 Ms then off 1 time
(I also have a wait action at the end of this of 2 seconds and then turn off just to insure the contacts are open before next use)