Here's an other way that you might find easier:
- Open a Terminal window
- Drag and drop the icon of SweetHome3D program on Terminal window to copy its path
- Type a space in the Terminal window
- Drag and drop the icon of the .sh3p plug-in file on Terminal window to copy its path
- Press <enter> in the Terminal window
This should run a command like: that will install the plug-in at the right place.

[Edit: don't use the -open parameter]