Sign in.
The app itself needs no account, and never will: it runs on your PC with nothing reported back. An account here is only for this site, and there are two things it does. It lets you send a message without a form that anyone can post to, and it shows you what your account is and when you made it.
Your password is stored only as something derived from it, and the cookie you are given is one this site alone can read. What is kept, and what is not, is on the privacy and licence page.
Sign in to your account
The address you signed up with, and your password.
There is no password reset by email yet, because there is no mail sender behind this site. If you have forgotten the password, use the contact page and say so, and the account can be sorted out by hand.
A wrong password and an address with no account are answered with the same sentence, deliberately. A page that told the difference would be a way for anyone to find out which addresses have an account here.
No account yet? Make one. It asks for an address, a password of at least ten characters, and nothing else.