OK, let's start at the beginning. Garmin devices I have used do not put out NMEA
data sentences, until you tell them to do so. OpenCPN looks for NMEA
 Have you set your Garmin EMap to output NMEA data? If not, go to the Interface Settings page (see p28 of the attached EMap manual), and set up NMEA output. See if the Baud Rate is stated or selectable (the OpenCPN default is 4800; OpenCPN can also receive at 9600 and 38400).
As I read your manual, your cable is set up using standard serial
port connections. So wiring
between EMap and PC should not be an issue.
 Do you now know the COM port your computer is receiving the data on? As I understand your last post, you found this in device manager or some other program.
 Go to 'ToolBox' in OpenCPN (spanner icon on top menu bar). When ToolBox is open, click GPS (second tab from left). Set 'NMEA Data Source' to correct COM Port (first data entry window from top), and set 'NMEA Baud Rate' to correct rate (third from top). Click OK.
Does OpenCPN work now? (Plugging the Garmin cable into a different USB socket can change the COM port, so begin by using the same USB socket all the time.)
Again, apologies if this is off track. Just trying to help ...