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
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.