Arduino Nano not visible in Serial Ports (Mac OS)
Is it an original Nano or a clone?

The original Nano uses FTDI's FT232 ship, whereas clones use the CG340 USB-to-TTL chip. The former works fine straight away Mac OS Sierra, whereas the latter needs a driver.

The CG340 driver doesn't work under Mac OS Sierra. Fortunately, someone patched the driver and made it available at GitHub. There you'll also find some troubleshooting instructions that can help (hopefully).

From Adrian Mihalko's repository at GitHub:



ch340g-ch34g-ch34x-mac-os-x-driver

Latest macOS Sierra-compatible driver for devices using the CH340G, CH34G or CH34X chipset. This chipset is used in several Arduino-clones.

Introduction

Version 1.3 (2016-09-27) of the OEM driver for the CH34x chipset currently causes a kernel panic (a.k.a. crash) when installed on macOS Sierra. To resolve this issue, please download and install the driver in this repo.

Installation


Remove the old driver by issuing one of the following commands (depending on your installation):


sudo rm -rf /System/Library/Extensions/usb.kext
sudo rm -rf /Library/Extensions/usbserial.kext

Restart your Mac.
Double-click on the CH34x_Install_V1.3.pkg file.
Restart your Mac.
Plug in your device. It should now be listed under the /dev directory. Examples:


/dev/cu.wchusbserial1410
/dev/cu.wchusbserial1420



Troubleshooting

If, and only if, the device is not recognized after the installation (or you cannot install the driver), please disable System Integrity Protection:


Reboot your Mac into Recovery Mode by restarting your computer and holding down Command+R until the Apple logo appears on screen.
Open the Terminal (Applications > Utilities > Terminal).
In the Terminal window, type in csrutil enable --without kext (or to fully disable: csrutil disable) and press Enter.
Restart your Mac.


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Regards,
Adrian Mihalko