Toward the bottom of the page, under "Macintosh:" click on the "I agree, Begin download" button.
You should get a DMG file. The Macintosh Download Manager should then download and mount the file then begin the install. If not, open the mounted file and run the installer if it does not automatically start installing.
Copy the firmware (D4_0104.bin) to the top level of your XQD card if it's your primary card, or your CF card, if it's the primary memory card. Make sure you copy the firmware to the top level (root) of your memory card. If you don't do both, your D4 won't see the file when you try to update it.
Drag the camera volume into the Trash and remove the memory card from the card reader or card slot.
Insert the memory card containing the firmware into the camera memory card slot
Turn the D4 on
Press the Menu button and go to the Setup Menu
Go to Firmware version and press OK
Go down to "Update" and press OK
Go to "yes" and press OK
Don't do anything else until the D4 tells you it's done.
Check what Firmware you now have to confirm the update's success.
If successful you can now format your memory card, then turn the D4 off.