Which version of OpenCPN
are you using?
The instructions for 3.2 are here Setting Options | Official OpenCPN Homepage
If you are on 3.0.2, have a look at the documentation
that comes with your installation
. Click the blue button with a "?" and then the help tab.
Tell us a bit more about what you have done and your setup.
If you are have gpsd running, for example, you have to connect to Network/GPSD (3.2), and not try a direct connection. More in the manuals
I suppose that "/dev/ttyUSB))" is a printing error, ought to be "/dev/ttyUSB0"
It would also help to have a look at your ~/opencpn.log file.