Place Lab is positioning software that works by having devices (i.e. laptops, PDAs, and cell phones) locate themselves by listening for radio beacons like wireless access points, cell phone towers, and Bluetooth devices. In particular, you may want to check out their documentation on the SerialGPSSpotter class, which uses rxtx to communicate with a serial interface GPS unit.
We recommend that you use the Garmin GPS device with a Windows machine, as there have been some reports of incompatibility with Macs. However, if you are just curious or really want to try it on a Mac, here are some useful links.