You must edit the default.php file to enter any custom URLs (anything you want indexed that isn't a Discussion or User). If you have User account pages disabled for guests, then change renderUsers to FALSE to prevent sending Google to pages it cannot reach.
It should be obvious that if your forum is not visible to guests, then this extension is pointless, do not install it.
Edit your robots.txt file to include this line to have your Sitemap auto-discovered: Sitemap: http://www.yourdomain.com/extension.php?PostBackAction=sitemap.xml
NOTE: THIS EXTENSION IS IN BETA.
For now this extension is in beta to test the concept. Updates will improve the structure and performance. I do not recommend using this extension on a large forum at the moment as the output can be dramatic and you will have created a denial of service method.
It is intended that a later version will periodically generate static gzip'd Sitemaps that paginate to other files at 20,000 URLs, when those are added and stable I'll update the version to 1.0 and it will be fine for large forum installs.