Seems like you are actually not referring to my plugin, but instead the PHP 5.2 compatible clone of my plugin, http://wordpress.org/plugins/widget-importer-exporter/. I can tell because of the custom file extension (.wie) you listed. My plugin still does use json.
When in Plugins > Add New – Search for “Widget Settings Importer Exporter” NOT “Widget Settings Importer/Exporter”
Removing the “/” will give you the correct plugin to install