Some notes on this process on the Mac
To install the Ardunio IDE on the Mac download the Unified firmware from TH3D.
Inside is a folder called MacOSX Arduino. Two zip files are present, arduino-1.8.5-macosx.zip and TH3DArduinoLibraries.zip
First unzip the arduino-1.8.5-macosx.zip and place the Arduino.app into /Applications
Then unzip TH3DArduinoLibraries.zip. this will result in 3 folders inside the main TH3DArduinoLibraries folder. You need to replace folders in /Applications/Arduino.app/Contents/Java/hardware with these 3 folders.
Then Follow the TH3D instructions, but com port will just show as /dev/usb and then some board ID and make sure the Programmer is Ardunio as ISP
You will need the install the U8glib lib in the IDE to compile Marlin. This can be added by going into following menu
Sketch/Include Library/Manage Libraries and filter on U8glib.