Immediately after login you will see a lobby screen which shows a list of servers you can join. The server list you see is directly related to the project your client is based on. You will not see or be able to join servers related to other projects. To join the 3D world, simply click the ‘JOIN SERVER’ button and wait for the 3D world to load.
If you can’t see a server (you see red text saying the server could not be located), either there is a network error, or there really aren’t any servers running at that time.