X11 forwarding is disabled by default. To enable it, modify the sshd's configuration file /etc/ssh/sshd_config as the following:

sed -i 's/#X11Forwarding no/X11Forwarding yes/' /etc/ssh/sshd_config

And then reload OpenSSH daemon:

/etc/rc.d/init.d/sshd reload

The current connection may not survive to the daemon's reload...