If you have an account setup but are seeing a sign up page after you login it means one of the following:
Did you sign up with a different email?
The email you are logging in with is not recognized as an existing subscription. Please check your invoice to find the email you used.
Did you recently change your login email?
If you had changed your login email, your previous email will not work to login.
To get back to the login page, click the "Log in" link at the bottom right of the sign up page: