The process can be split in three big steps:
In more detail, it goes like this:
You do not need that driver if you are using bluetooth.
You will need to perform the following operations once. The operations make sure that a lock mechanism that is needed by RxTx can work for any user:
Most of the time this will make things work. You may still get trouble if you have a 64 bit operating system.
Indeed, at the time of writing, the only RxTx driver available for MacOS is a 32 bit version. You might be running a 64 bit operating system. Your 64 bit operating system can still run 32 bit programs, but when running a 64 bit program, all code it uses 'directly' must be 64 bit. As the RxTx driver does not come as a 64 bit version yet, running Java as a 64 bit version will not work. Quite franckly, there is not real reason to use the 64 bit version of Java, but you probably are if your operating system is 64 bit.
The solution is to install the 32 bit version of Java instead. I did not look where to find it, send a note to the application author if you know where it is.
When using the Web Start system (running BT747 from the web) the above considerations are still valid. On top of that you also may need to give permission to the application to allow it to use the system's resources.