I had not seen that page (and the ReadMe file)
I went to PlugIns | Official OpenCPN Homepage
which seems to be the official opencpn
download page for plugins and the ReadMe file doesn't exist there nor is there a link to the page you quoted.
Ahhhh ... found the link in the 'Source code' section which is about the only section I didn't explore since I don't know enough to build from source
If you have access to the plugins page, maybe you can add a readme file which has that info as well as where to stick the downloaded files. I think that would be a great help for those of us using the pre-built files.