OK I see. In 1.5.8,

$fs->loadFilesFromPluginsDirectory($installedPlugins, 'admin/includes/extra_configures', '~^[^\._].*\.php$~i');

was added to ./admin/includes/application_bootstrap.php. I will have to check to see if this is also intended to be added to 1.5.7. For now, you'll have to add this line manually to that file. (right above the load of extra_datafiles)