Check the first line of the three config.core.php files (MODX root, manger, and connectors directories).
It needs to point to the core directory and end in a slash. Here's mine for XAMPP on localhost:
define('MODX_CORE_PATH', 'C:/xampp/htdocs/addons/core/');
If that doesn't fix it, check the paths and URLs in the core/config/config.inc.php file.
