I know this may seem somewhat basic, but please check your admin directory and verify that it has a file named googlesitemap.php.

If it does, I would consider walking through the simple installation instructions again and check to verify that all the included files were uploaded properly. If it doesnt, you need this file and missed it during installation.

Can you verify that the SQL was updated? Did you complete step #2 of the installation which stated
2. Load sql path;