OK, first things first. And when having Serial-Device-To-OpenCPN problems the first thing to do is check that you are actually getting any data from the device.
Use any Serial Port monitor
application to connect to the port and look at what data is arriving. If you don't have any such application, a really simple one to get is PuTTY [Download
Connect to your devices (Speed can be 4800 or 38400). If the thing is on and working you should see a stream of data coming in in the window. If nothing happens you either are looking on the wrong port, or there is a problem with your device or its driver or its wiring