Empty referer - no referring page. That means the file has been called directly or you are using software that prevents passing the HTTP referer (browser, firewall).
Note: Once the script configuration has been finished you can set off these system messages (index.php).
Run into problems? Script documentation and instructions: ./docu/index.html
Get answers to your questions in the support forum at the website of the File Download Script on GentleSource.com.