You must have an account with Mojang.
If mc.livingcomputers.org does not respond from a browser, then the server is DOWN.
A map is available at mc.livingcomputers.org:8123/
This will be your opening screen when you start Minecraft on your computer:

Choose Multiplayer

Choose Direct Connect

That should get you to our Castle at Python World

If you will often be using this server, or if you use multiple servers, you may wish to add livingcomputers to your server list

For Server Name you may wish to be more descriptive

Your Miltiplayer Opening Screen should now show