It's a serial device. If your PC has a serial port then put the USB adapter lead away and use the serial cable that came with the GPS. If you don't have a serial port then you MUST install the USB adapter driver. Some USB adapters work (prolific U232, pfranc, garmin), some don't.
Now go to the Garmin website and do a firmware update. Garmin use a pretty resilient app to do this and it will tell you which COM port you are connected to. If the garmin firmware updater can't work it out then you have bigger problems.
Now you know the GPS is configured OK and you know the COM port. Enter these into your mapping software and disco. If you are using TrackLogs then download 3.14 beta as it automates all the GPS settings. If you are using something else then shame on you, ask them instead.