If you use the free version, imported stuff and preferences can be found in the folder Library/Application Support/eTeks/Sweet Home 3D of your home folder (you may use Go > Go To Folder...menu item in the Finder to go to that folder, adding the prefix ~/).
If you use the Mac App Store version, imported stuff and preferences are stored in the folder Library/Containers/com.eteks.sweethome3d.SweetHome3D/Data/Library/Application Support/eTeks/Sweet Home 3D.

See also this post.