From the user Adam Venturella.

10.13 has a new restriction on kext like the others have said. If you want to manually load it:

sudo kextload /Library/Extensions/SiLabsUSBDriver.kext/

You will then see the security error:

/Library/Extensions/SiLabsUSBDriver.kext failed to load - (libkern/kext) system policy prevents loading; check the system/kernel logs for errors or try kextutil(8).

Now, from your System Preferences, go to Security & Privacy at the bottom of that window you will see OSX is asking you to approve the kext from Silicon Labs. Do that and you should now be able to see the device:

ls /dev/cu.*

...
/dev/cu.SLAB_USBtoUART
...

Good luck :)