The solution? Download something called XQuartz.
Step 1: Download Freeciv & Xquartz.
As I said earlier you will need to download something called Xquartz, thankfully this is also free and is available here.
Step 2: Install Xquartz.
When you do YOU SHOULD READ WHAT IT SAYS (Never install something if you don't agree with the terms, how many of you actually read your email EULA?) and press continue. Read what it says again and press "Continue" again. Read what it says again and press "Continue" again.
It will now ask you if you agree to the terms of the software license agreement, IF YOU DON'T, THEN DON'T PRESS "Agree"s If you do, then press "Agree" and then "Install". You will have to enter an administrators name and password, then you can press "Intall Software".
Now it should be installing. Press "OK" when it says it's finished.
When it is finished, it will tell you that you need to log out then back in for it to take effect. I suggest you do so.