Hi, here's some tips for playing multiplayer!
Playing over the internet
Unfortunately, you need to port forward if you are the one who is hosting. If you don't know, that means you need to set your router to allow all networking traffic from your specific computer and port.
I can't give you specific instructions since it varies per router, but essentially, you have to go into your router settings and add the game's default port (or whatever you changed it to), and possibly your computer's internal IP address, which is accessible via command prompt/terminal.
Google will tell you how to find your internal IP and how to port forward your router. The default game port is 6675. In the future you will be able to change the default port. I'd rather get multiplayer working properly before adding frivolous things such as that.
If you are the one connecting, you'll have to ask the host for their external IP (Google can tell you what your external IP is), and type that into the main menu screen.
If it's not working try opening the port in your computer's firewall, and make sure Java is allowed, also.
Playing over LAN
If you are hosting a game for folks beside you, no port forwarding is required. Just open a game and wait for them to connect.
If you have any questions, shoot me a message or reply to this thread.
Thanks!