What is the sitemap location?

This may seem like a silly question but in setting up the Google webmaster Tools section for Harry Potter Chat, I am asked for the sitemap file location. I tried the root and after an error there was a report saying the file format was incorrect. I also tried Harry Potter Chat and was told

The Sitemap must be located at Harry Potter Chat. To add a Sitemap at Harry Potter Chat, first add that site to your account and then click the Add a Sitemap link beside it.

I believe everything is at the default - what location should I have used?

its really in vbseo_sitemap/data/sitemap_index.xml(.gz)
but there's a re-write to make it appear in root.

address it like site.com/sitemap_index.xml(.gz)

Perfect. /sitemap_index.xml.gz was the key.