This is for those who are really new to Ubuntu and is trying to install this plugins. The official guide currently available is too simplfied and not helpful to newbie. So here it is:-
1. Unzip the downloaded plugins files RotateHomePlugin.zip, about 13.1kb in size.
2. The unziped files will produce a folder named RotateHomePlugin, with many subfolders and files in it.
3: Open this folder and go to the subfolder RotateHomePlugin/dist/, and COPY the zip file named RotateHomePlugin.sh3p (yes, it is a zip file, and don't unzip it)
4. Now you want to PASTE this file into the plugins folder in this directory Home/.eteks/sweethome3d/plugins
5. It is possible this directory is hidden, and to see/unhide it, click the Home folder and press Ctrl + H together.
5. If there is no plugins folder in this directory, please create it.
6. Now, PASTE the RotateHomePlugin.sh3p zip file into the plugins folder. Remember don't unzip it.
7. Restart the the Sweethome3D software, and you should find two new menus under the Plan menu called Rotate clockwise/counterclockwise.
Hope that is helpful to someone
Gan