OpenRA

Un/fortunately this issue isn't limited to OSX.

Servers must forward the port that they are hosting on in order for people to be able to join it. We don't say this in big flashing letters anywhere, so most people aren't aware of this requirement.

If you can find a server that is set up correctly, you should be able to join it.

you have to go to the webpage of your router, usually 192.168.0.1 or something along those lines. Then find port forwarding, usually under NAT/Qos and enable forwarding for port 1234.