What is the url of the sitemap you are submitting?
Have you generated the sitemap.? Have you performed the .htaccess file edit?
you should submit something like
http://yoursitename.com/forums/sitemap_index.xml.gz if your site is in the /forums/ folder or
http://yoursitename.com/sitemap_index.xml.gz if your forum is in the webroot.
The best place to ask for modification support would be the modification thread, the developers will likely see it and help you more