This is covered in a couple threads on the
openCPN board. The bottom line is that Furuno units require at least one specific sentence to end all transfers, and openCPN currently does not support it. So currently you cannot use openCPN to directly upload/download to/from a Furuno
GPS or plotter. Support has been on the developers to-do list for a while, but it's low priority.
As an alternative, you and can use GPSUtility (
GPS Utility - Home) to transfer waypoints, routes, and tracks. gpsu will read and write GPX files, so you can use the GPX
Import and Export facilities in openCPN, and then use gpsu to communicate directly with the plotter. The free version of gpsu limits your waypoints (100) and routes (5); The licensed version ($60) allows numbers that are practically unlimited.