I cannot really help you much with the Mac, as I am very inexperienced with that OS, but surely they do have a firewall in there, right? Or by "which are not using a firewall" did you mean that you already disabled the firewall on the clients?
By open ports I mean allowing traffic from outside to the specified port numbers on the computer. It's a firewall configuration thing. And yes, if you don't have any firewall on any of the machines, it should work...
One more idea that comes to my mind is - are you on the same subnet on both of the machines? Can the machines ping each other?
Also, the Licensing server has a log that should be able to tell you, whether the request from the render client arrived to the LS machine at all. Then we'll be able to tell where the issue with the communication happened.