Your username is the same as the email address you used to register, and that is what is displayed in the forums to identify you. But what if you don’t want to share your email address with everyone? We are still fixing the profile editor, but in the meantime here is a work-around.
Go to the bottom of this page, type a reply to this message (it doesn’t matter what you write), and click on “submit”. Now go to the bottom of the page again, find your message, and click on your email address next to the message. You will be taken to a profile page; at the bottom of the menu on the left select “Edit” to go to the profile editor. Now you can enter a nickname, and set that as your display name. Go to the bottom of the page and click on “Update profile” and you’re done.