If you have created the server but have forgotten the name, do /join own then right-click the book to find the name. To make it super-flat, install multiverse core using /control then restart the server. Once restarted, do the command: /mv create Superflat NORMAL -t flat Once you are there, do /setspawn to make it your server spawn.
Thanks for sending in the bug! I can always try and help people who need it. The things I suggest is just to make sure you are logged in, even logged into the right account. If not, Samerton hosts the website so it may be a good idea to contact him.