AmeriCommerce generates two types of sitemaps. One is a a .aspx page, meant to be viewed by customers and/or people on the internet. The other is an XML document, meant to be parsed by search engines like Google or Bing.
Sitemap for Humans
To get to the easily readable sitemap, meant to human eyes, you will want to go to the path /store/SiteMap.aspx on your website. Here you will see a sitemap, that also has a theme level page should you decide to make any changes to its layout or how it looks.
Sitemap for Search Engines
The other sitemap is located at /sitemap.xml on your website. When you go here, it will generate an XML document on the page, which will list all your products and categories. This is something that you sometimes need to submit to search engines, so they can quickly map the site.
Generating a Fresh Copy of Sitemap.xml
If you ever need to download the XML version of the sitemap, you can go to /store/SiteMapGenerator.aspx on your store. This should download a new document straight to your pc.
Alternatively, if you've added new products/categories and the XML sitemap hasn't updated yet, you can simply delete the sitemap.xml and sitemap_index.xml files under Contents > Files. After deleting the files, simply go to /store/SiteMapGenerator.aspx and the new file should be generated on the server.