@meltdown:

I observe you are running v.1.34. I suggest downloading the just released v.1.35 which comes with web-based installer. (install_googlesitemap.php)

Regards to your error above, it appears that GOOGLE_SITEMAP_XML_FS_DIRECTORY is undefined. Are you certain you uploaded all files from the 1.3.4 version release and overwrote all files from any previous version(s)?

As suggested above install the new 1.3.5 and run installer. Before installing you will want to decide where you install your XML files, in your webroot (opublic_html) or shop root.

And then after installing double check admin setting for "Use root directory" in Configuration > Google XML Sitemap

Woody