After installation, I tried posting a test comment as a normal guest. The comment was not posted, and I got the following message:
"Unable to include wp-config.php. Please make sure 'captcha-free.php' is uploaded to the 'wp-content/plugins/' folder"
The plugin was installed via WP Admin directly from wordpress plugin repository.
I had a look via FTP, and the plugin folder is there, and contains 3 files: