To sign a message - click on receive money - click on actions - click on sign message - type the message you want to sign (see below for what I want you to sign) - click on sign message - enter 2nd password as necessary - reply with the signature you are given
The address you are signing from must be somehow associated with your account previously. I checked your post history and could not find any signed addresses. If you had sent a PM with an address in the past then I can use that address to confirm that you own the account (you would sign a message from that address).