To join a Server enter the Adress in the TextBox named "Server Adress" enter your Username in the TextBox below the "Enter your username below"-Label and click on "Login".
After that a Welcome message should be shown in the Output in the middle of the window. If thats not the case check the Address and check your Firewall.
After you are connected you can enter your Message in the Message-TextBox and send with the Enter-Key