I have just installed and activated WP-SimpleViewer v1.5.4 (available from here) and have successfully managed to create and view a gallery so WP-SimpleViewer v1.5.4 does seem to be compatible with WordPress v3.4.
Have you made sure that your gallery's fodler has been given full access permissions (777)?
Also, when creating the files for the gallery folder, do you get any error messages or does it say that it has been a success?
If checking the gallery folder via FTP, is a 'gallery.xml' file present?
Please post the URL to your gallery so that I can take a look.
SimpleViewer Support Team.