Maazito, here's the exact structure of user files handled in Sweet Home 3D that you should back up under Mac OS X:
- In your home folder, preferences are stored in the com.eteks.sweethome3d.plist file found in the Preferences subfolder of the Library folder (the latest folder may be renamed as Bibliothèque or an other localized name if your system is not in English).
- Furniture and texture files, and other libraries are stored in the Application Support/eTeks/Sweet Home 3D folder found in the Library subfolder of your home folder.

This doesn't include Sweet Home 3D program itself that you can copy in Applications folder or anywhere else on your disk.

If you copy at the same place the users files on an other computer, it will work only if the user name/login on the destination computer is the same. Thus, you'll probably prefer to copy the furniture you added to your catalog, with the Export to SH3F plug-in available at http://sweethome3d.eu/plugins/ExportToSH3F-1.0.sh3p.
Download the plug-in on your home mac, double-click on it to install it in Sweet Home 3D, relaunch Sweet Home 3D, choose Furniture > Export imported furniture to SH3F... menu item, copy the created SH3F file on the other computer, and double-click on it to install it there.