I don't know about the plugin but i always use this website for sitemaps. Super easy http://www.xml-sitemaps.com/
By default this plugin stores your xml file in plugin's folder. but it has option to store xml file in your website's root folder. So, it is either here: yourdomain.com/sitemap.xml or here: yourdomain.com/wp-content/plugins/simple-google-sitemap-xml/sitemap.xml
First open this site http://www.xml-sitemaps.com/ then Enter your site domain then click on generate after generating site map open google webmaster tool and upload your site map when you upload site map .
Google xml sitemap will automaticly generate 2 sitemap file on your domain, sitemap.xml and sitemap.xml.gz you can check your sitemap file like this >> your-domain-name.com/sitemap.xml