See if this discussion helps http://www.mp3car.com/wiki/index.php/GPS.
First, make sure the driver is installed properly. Plug the GPS in and then go to the About This Macintosh selection under the Apple menu. Click 'More Info' and the System Profiler will launch. Look under USB for your receiver. If it's there, that's a good sign.
Then, make sure you have activated the port correctly. This is probably where you've gone wrong. Open the Preferences-->Network control panel with the GPS plugged in and you should be informed that you have a new port available. You have to select "Network Port Configurations" from the drop down and check the box to activate it.
Since the Mac doesn't have serial ports, what it does is mimic them virtually. The driver you installed will write data to and from the GPS device to a file located at /dev/tty.serial1 or some similarly named file (it depends on the name of your GPS device, which you can get from the Network Port Configurations).
Then, get an app called GPSUtility for the Mac. It will give you a drop down of the possible devices. Pick the one you think it is and open the GPS window. It will show the actual data streaming in from the GPS. If you can't get it to work with that, then the installation of the Holux driver or your network port configuration isn't working properly.
Once you get it to work with GPSUtility, it ought to show up in Roadnav. I think you'll have to select NMEA mode for the Holux.
After you've mastered that, look into using GPSDx so you can share that receiver with multiple applications. All you'll have to do is install GPSDx, then launch the ConfigGPSDx application that comes with it. You specify your GPS device and start. It will tell you if it's working properly.
Then you go back to Roadnav and select "Use GPSD" in the GPS options. Do the same with the other apps that you want to use