copy the IP then type /connect (IP address) or just double click on the server... easy. first type ` (usually next to number one left of theyboard. Then type /connect IP:PORT, press enter and if it is public you should gain entry.
When you play offline, the only 2 modes you can play are Multiplayer and Zombies. So when you log on you'll see the 3 modes. Multiplayer, Blackout, Zombies. Just click on the word (Multiplayer) or (Zombies) under their respective pictures and you can then set up the match how you want.