Aart:
rtl_ais is an external program, no need of the rtl-sdr
plugin.
Run it from a command line or a batch file. The ppm error correction parameter is a must. Check again the readme file.
Use also the parameter -n, and if all is working (a proper antenna,correct ppm parameter, AIS signal presence), the AIS sentences will be appearing on the console window.
Then in
OpenCPN, if doesn't already exist, you should create a
network connection type UDP port 10110, that way opencpn will receive messages from rtl_ais.
I'm aware this is not easy if you are not familiar with
software defined
radio and command lines, but if you want to spend some time trying, without warranties of success, I can give you some directions may be can help you.
David