UPDATE: THE MASTER SERVER IS NOW UP AND RUNNING! You should now be able to connect and play with each other.
I could not justify paying for keeping the master server (which connects players to each other for online play) up, so it expired. I am planning to put up a new one some time in the near future though, but in the meantime, local play is the only way to go. I'll let you know once the server is up again!
Thanks for playing!