To make sure this is not misunderstood: this is not a bug in the game or the master server. Instead, it means that it is not currently possible to connect to your computer from the Internet. But that's neccessary if you would like to host a game (and I hope you do, since if nobody hosts, nobody can join). So you need to setup port forwarding on the device which your Internet provider gave you to access the Internet. Those are usually referred to as "DSL modem/router" or "cable modem/router". Read on in the FAQ for a more detailed explanation.
