Ive been digging around a bit more and I can see the following:
1) Google XML Sitemap plugin can only be seen in the plugins menu on the network site.
2) Once activated, it provides the following link to the settings from the plugins list: http://e-commerce.ph/wp-admin/network/options-general.php?page=google-sitemap-generator/sitemap.php
This address is naturally incorrect as it produces a 404 error. Google XML Sitemap plugin can only be accessed via the base site and all other sites in the settings menu, but not from within the network site where it links to.
I know this is a beta, but it seems to have so many issues, I may have to switch to an alternative so that it creates a sitemap as I need it for the search engines. I have used your plugin for years upon years and I hate to change, but needs must.