On http://memn.io/app.html (you won't see the link for some reason if you have Ghostery installed), you should be greeted with a sign up page [1]. If you have already made an account, you can click the "Sign In" link in the top right corner to switch to the Sign In form.

If that's not working, I'd appreciate it if you could email [email protected] with your browser/version/OS.

[1] (http://imgur.com/jQeY60N)