Prerequisites to run X applications on OS-X:

You need to have XQuartz installed on the client side (check that it's working for your account by running XQuarts and starting a local terminal)
You need to allow XForwarding on your server. Set the X11Forwarding option (most commonly found in /etc/ssh/sshd_config): X11Forwarding yes (don't forget to restart the ssh server after editing the sshd_config file).
Your sshd server should be running on the server side.
Your firewall (on both ends) should allow for traffic over port 22.