The sitemap plugin does not create a static file. It renders the sitemap on the fly when Q2A receives a request. The correct path depends on what you configured as URL structure in the admin settings under "General."
For example, on my own forum I'm using the second option from the top, so for my site the URL looks like this:
www.example.org/index.php/sitemap.xml
If you chose the 4th option from the top your URL would probably look like this:
www.example.org/?qa=sitemap.xml
A URL www.example.org/sitemap.xml will only work if you selected the first option, which does not work with all webservers.
Something you can do to make the sitemap accessible directly under the domain is to configure a location on your webserver that maps www.example.org/sitemap.xml to the correct URL structure for your forum configuration. You need admin access to the webserver for that, though.
For further help please show what exactly Google is complaining about.
