For OS X Mavericks (10.9) Apple apparently developed their own drivers for
the FTDI chip which donıt exactly work. So the solution is to disable those
built-in drivers and install the drivers available from FTDIıs web site.*
The procedure is documented he
http://forum.arduino.cc/index.php?topic=198539.0
Note that the drivers on the FTDI download page show latest compatibility as
Lion (10.7) but reports are that they are working with Mavericks and
Yosemite.*
FYI