To ensure you created the directory at the place, try this tip:
Import an existing SH3F file with Furniture > Import furniture library... menu item.
This will create .eteks/sweethome3d/furniture subdirectory in your home directory with the imported SH3F file in it.
Go to the .eteks/sweethome3d directory, create a subdirectory name plugins in it and copy the ExportToSH3F-1.0.sh3p file into plugins subdirectory.
Finally, relaunch Sweet Home 3D.