After a year, the page still says you can use your screen name, but answer#1 is right:you have to use your email address. Imagine the lost time of the 9578 people who needed to view it.
Actually you can use both, but as your email address is the single detail that cannot be duplicated on site (unlike your screen name), it is probably advisable to use the email address to sign in.
Thanks for clarifying!
