I have only been able to find references to this prblem on Linux. I have installed the latest FTDI drivers but that hasn't solved the problem.
I am running Mac Mountain Lion 10.8.2 which is the latest release. This is running on a Mac Mini with an Intel Core 2 Duo processor. There is 8gb of memory on board with 5.88GB memory free and plenty of disk space. The FTDI drivers downloaded and installed are noted on the dmg file as v2_2_18.
When I run the Arduino software, the Serial Port menu option is greyed out and I get the following error message in the Arduino window:
java.lang.ClassCastException: gnu.io.RXTXCommDriver cannot be cast to gnu.io.CommDriver thrown while loading gnu.io.RXTXCommDriver
The board is working fine because I have installed the Arduino software on my Windows 7 laptop and have uploaded sketches etc. without trouble.
Any help would be really appreciated.