Hello and Welcome! to join multiplayer is really easy you just go to the tab that says "Multiplayer" and click it then if you want to add a server too keep it on your list click "Add server" just type the server name and IP, The Ip is what you need to connect to the server just copy and past it with this commands copy= crtl+c
paste= ctrl+v (If you have a mac i'm sorry I don't know the commands) or you can just type it no problem! then you done and you can play now! or you can just click direct connect and you just type or paste the IP and you're done but this will not add the server to you're list.