An ownership or permissions problem is preventing the system from making the device accessible.
Check that the driver is owned by root and wheel. The most common symptom is the group for FTDIUSBSerialDriver is not wheel. To change the group, login as root and perform the following script in a Terminal window (Go > Applications > Utilities > Terminal):
chgrp -R wheel FTDIUSBSerialDriver.kext
Reboot for the change to take effect.