Usb Serial, Mac os and a Cisco switch

List device names

 ls /dev/cu.*

A list similar to this should show up.

mba:~ root# ls /dev/cu.*  
/dev/cu.Bluetooth-Incoming-Port /dev/cu.Bluetooth-Modem **/dev/cu.usbserial**

connect using cu

cu -l /dev/usb.serial -s 9600

If you do not see a usb serial device, you can list all your usb devices with this command

system_profiler SPUSBDataType

Which in my case showed my Prolific usb adapter

USB-Serial Controller D:
 
  Product ID: 0x2303  
  Vendor ID: 0x067b (Prolific Technology, Inc.)  
  Version: 4.00  
  Speed: Up to 12 Mb/sec  
  Manufacturer: Prolific Technology Inc.  
  Location ID: 0x14200000 / 6  
  Current Available (mA): 500  
  Current Required (mA): 100