Usernames do not have to be e-mail addresses. We do not require usernames to be e-mail addresses on purpose so that if two or more people share an e-mail address, they can still have separate Team Cowboy accounts. That being said, it's perfectly fine to use your e-mail address as your username if you wish.
If you are not sure what your username is, you can request an e-mail which will help you figure it out. Just go to the Retrieve Account Information
page and follow the instructions on the screen.