Use:
http://www.yoursite.com/forums/sitemap_index.xml.gz
(It should work in your browser too, if you go to it, as long as you made the .htaccess changes instructed.)
It will automatically notify Google of your updated sitemap (if you have that option turned on), but it won't show up under your Google Sitemaps account. It's done "behind the scenes". You will have to manually add it if you want to see/track it under your Sitemaps account..