Why so complicated?
Just download to ~/.eteks/sweethome3d/furniture and start SH3D. The sh3f file is a zip archive which can be extracted by SH3D when running.
Same for the textures and plugins to their respective folders.

On Windows the path is (when installed on C:) C:\Users\<username>\eTeks\Sweet Home 3D

I keep a backup copy of the original file because I remove furniture/textures I don't need with the editor from the libraries.