Where is Arduino.h on a Mac? Can I download a copy if missing?
On a Mac the Arduino.h file is within the Arduino application itself. It's not that easy to get to without resorting to the command line.

You also need far more than just Arduino.h - you need the entirety of the Arduino AVR core, which is a collection of .h, .c and .cpp files. They are all together in the application.

You would probably be best off getting them from the github repository (https://github.com/arduino/Arduino) - the files you need are all under hardware/arduino/avr

On your Mac you may find them as /Applications/Arduino.app/Contents/Resources/hardware/arduino/avr or somewhere similar (not entirely sure where it puts it within the app).