Modifying Print.h on a Mac
Each hardware package has its own version of Print.h in the cores folder. The Arduino IDE installation includes a copy of the Arduino AVR Boards hardware package but that package will not be used if you installed a different version via Boards Manager (Tools > Board > Boards Manager) or if you have a board selected from the Tools > Board menu that is not part of the Arduino AVR Boards package. Operating systems seem to like to hide the folder that Boards Manager installs the hardware packages to by default, which makes things more confusing.

The easiest way to find the location of the active hardware package, and thus the Print.h you want to edit is this:


Select a board of the hardware package you want to locate from the Tools > Board menu.
File > Examples > SPI > BarometricPressureSensor (or any other example sketch of the SPI library)
Sketch > Show sketch folder - this will open the BarometricPressureSensor example sketch folder.
Navigate up 4 folder levels to the architecture folder.
Open the cores subfolder of the architecture folder.
If there are multiple folders inside the cores folder you will need to open boards.txt (which is located in the architecture folder) and find which core library your board is using, which is specified by the build.core property. Most hardware packages only have a single core library so this should not be necessary.
Open the core folder. There you will find Print.h.


The reason I selected the SPI library example is because this is a hardware specific library that is bundled with all hardware packages I've encountered. If you encounter a hardware package that does not bundle an SPI library you would want to open the example for another hardware specific library such as Wire, SoftwareSerial, or EEPROM.