I have Read the plug in page. I have dbl clicked on the .sh3p files, select "run with another application" and select sweet home app. I then get a pop-up telling me that the plug-in was successfully import and will be available on next launch. The good news is that I have a Tools menu now, but only 1 plug-in shows.

I have installed 6 plug-ins this way, but only 2 shows on the Tool Menu.

You already have a library import that seems to work, why not create a plug-in install routine also. It has to be almost the same.