If you forgot your username - you can enter ONLY your e-mail address and if you have an account registered with this e-mail address you will get your username and password mailed to this address.
If you don't remember the e-mail address but know your username - you can enter ONLY username and you will get your username and password mailed to the correct e-mail address.
If your password was found and mailed to you successfully - you will be taken to main members page. Wait for the e-mail with password and login to your account manager.
Otherwise you will see an error message with details explaining what you did wrong.