I found a couple of things are needed for the Sitemap function to work:
1. Enable Friendly URLs:
Go to System Settings
In the "Search by key..." field type: friendly_urls (Hit enter)
Set the value for "Use Friendly URLs" to: yes
2. Have the .htaccess file in the webroot:
Rename the provided file ht.access to .htaccess
3. Make Apache use the .htaccess file:
Edit your httpd.conf/httpd-vhosts.conf file and find the host/virtualhost of your site
Add the Option "AllowOverride All" to the Directory definition:
<Directory "/var/www/my.modx.site">
AllowOverride All
</Directory>
4. Restart/Reload Apache
5. In Resources click "New Static Resource"
On the "Static Resource" pane:
Title: Sitemap
Uses Template: (empty)
Resource Alias: sitemap
Static Resource: [[!StercSeoSiteMap]]
Published: Yes
On the "Settings" pane:
Resource Type: Document
Content Type: XML
Rich Text: No
Cacheable: No
Empty Cache: Yes
Save
Now browse to
http://my.modx.site/sitemap.xml
Best regards
Michael