Apple's MacOS X 10.6 version that goes under the codename "Snow Leopard" is no longer compatible with the official stable driver for the serial link (RXTX).
The more recent "Beta version" of RXTX has to be referenced and that version is RXTX 2.2Pre.
For the moment, the easiest way to get BT747 working on Snow Leopard is to use the webstart version that is labeled as "BT747 Latest using Rxtx 2.2pre".
The change can be integrated in the other releases of BT747 but I'ld like some volunteers to validate the changes and if possible even actually develop the changes in the startup script. You can propose yourself in a comment to this note or by sending me a message (feedback form, mail, ...).