Starting with Mavericks (Mac OS X 10.9), Apple has included FTDI drivers with their system. As such, if all you're using is vmAIS, there's no need for a separate install of the FTDI driver anymore. For Mac OS X 10.3 to 10.8 you will need to download and install a separate FTDI driver, which can be found here.
If you happen to have a separate FTDI driver installed and loaded, and try to access the COM port can cause a system crash. Yuck!
To fix this you can run the attached script. To run the script "right click" (click with two fingers) -> Open With -> Other... -> Utilities (down the list) -> Terminal.app. The attached pdf describes the steps being performed by the script and also provides the instructions for manually fixing the issue.