You must fill in the authentication at the top, just below the files menu. There should be places for your host, username, pasword and port if needed, usually that is port 21. After you fill in that, just press enter. After you have successfully signed in, drag something from your computer to the server!
Make sure to select the passive mode option if you have broadband and use a cable/dsl router. That stumped me for about an hour because I thought the password was bad, but I had just reinstalled Fetch and forget to change that setting. Woops :-)