Usually the way these drivers work is that they create what is called a USB to serial bridge. (Explanation: here) That means you have to activate the serial port before using it. You do this from the Preferences-->Network panel. Under the "Show:" dropdown, select 'Network Port Configurations" and activate your GPS port. That will make it visible.
Try Roadnav then. If it does not auto detect your GPS, Google for a program called GPSUtility for OSX. It gives you a simple screen that allows you to specify your GPS connection and see the data coming across from it.
Also, consider installing GPSDX. This is an invisible daemon that runs in the background and allows you to share your GPS between programs that support GPSD. Roadnav supports it, Kismet wardriving software supports it, and G2GE, the gps to Google Earth software supports it. You can share your GPS info between all of these apps if you use their GPSD connection option.
I've used VNC with my Mini and it is pretty cool to remotely control it from the house. Just remember to shut it down or you'll have to jump the car the next day!