You didn't mention your OS...Windoze?Linux?or if it's a usb conected.(some of these are not)
Assuming Usb,in Windoze,you must discover the "Port"your gps is using.Device manager should have it listed.The port will be different when you plug
into a different usb jack.
I agree: OpenCPN -my new favourite! SeaClear is good too but like getting into a Russian tank for Windows slackers like me.
Have a look for XPORT too.What it does is help find the gps and pass along the info to the charting Program.Leave the gps on awhile if you are using it to FIND the port for you...XPORT even allows Multiple charting programs at same time and helps if you want to put the computer into "standby" because Windows alone tends to forget the gps on wakeup....