Please make sure your computer is not behind a firewall when you attempt your submission:

Firewalls are security measures installed in your network to restrict the sending and receiving of files to and from the web. Because you need to transfer files to our server during your submission, a firewall will interfere with the submission. Proxy servers can cause the same problems. If your are experiencing any problems completing your submission, please check with your system administrator to make sure you are not attempting the submission from behind a firewall or using a proxy server.