Well that was a very interesting few days! :stuck_out_tongue_closed_eyes: For a little while I thought that I was going quietly insane. :astonished: :stuck_out_tongue: But its all fixed now...
I received from an EBay seller a small batch of Arduino Nano ATMega328 boards and ran into this problem immediately. All of the symptoms described by other posters in this thread I experienced. Nothing worked. I must have installed the FTDI drivers dozens of times, but the Arduino Nano would not be seen by my system, a Macbook Air with OS X Mavericks 10.9.5.
It was another thread that gave me the clue - perhaps the Nano doesn't use an FTDI chip at all. And sure enough, it does not.
Etched on the chip are the details; WCH CH340G 2026850406 - so I searched Google for "CH340G" and located this thread at the top of the search.
The Chinese website is very slow and the download of the WCH drivers failed. But thanks to @rich1812 (above) the file he links to downloads very quickly and installs okay in OS X Mavericks.
Be sure to reboot the Mac after installing the drivers.
When plugging the Nano in to the Mac's USB port you can check if the driver has loaded by opening up a terminal and entering the command
My own device shows up as tty.wchusbserial410
So now I can get on with making use of these cheap Chinese Arduino Nano. Woo hoo!