//command [usb_port] [baud rate]
$ screen /dev/cu.SLAB_USBtoUART 115200
#screen /dev/tty.SLAB_USBtoUART 40000

Brief explanation of tty vs cu

NOTE: Use ctrl+A to make commands. Type ctrl+A followed by ctrl+\ and press return to exit the screen session. If you exit terminal without exiting screen the screen session will stay alive and the serial resource will remain unavailable until the screen session is terminated manually.

$ sudo ls -lh /System/Library/Extensions

You might see the following entries:

SiLabsUSBDriver.kext
SiLabsUSBDriver64.kext

If you have those and still things do not work, remove, and reinstall- if you don't want to remove you can always rename the files:

$ sudo rm -rf /System/Library/Extensions/SiLabsUSBDriver.kext
$ sudo rm -rf /System/Library/Extensions/SiLabsUSBDriver64.kext

List tty devices:

$ ls /dev/tty*

$ ls /dev/cu*

Or just list the Silicon Laboratories- SLAB- devices:

$ ls /dev/*SLAB*

You can also user system_profiler:

$ system_profiler SPUSBDataType

Which outputs:

 USB 3.0 Hi-Speed Bus:

      Host Controller Location: Built-in USB
      Host Controller Driver: AppleUSBXHCI
      PCI Device ID: 0x8c31
      PCI Revision ID: 0x0005
      PCI Vendor ID: 0x8086
      Bus Number: 0x0a

        CP2102 USB to UART Bridge Controller:

          Product ID: 0xea60
          Vendor ID: 0x10c4  (Silicon Laboratories, Inc.)
          Version:  1.00
          Serial Number: 0001
          Speed: Up to 12 Mb/sec
          Manufacturer: Silicon Labs
          Location ID: 0x14500000 / 9
          Current Available (mA): 500
          Current Required (mA): 100

You can also access this information from System Information (Applications > Utilities > System Information). Just look for the USB entry on the Hardware dropdown.