cd ~/Desktop [or wherever your browser is set to put downloaded things]
su [and type in your root password]
cp Darwin1.0a3.tgz /
cd /
tar zxvf XDarwin1.0a3.tgz
rm XDarwin1.0a3.tgz
For more details see http://mrcla.com/XonX/
Upgrading from Mac OS 10.0.4 to Mac OS 10.1 also overwrites the /etc/ssh_config file, so even if X-forwarding worked before it is now broken. Go back and put the following lines at the end of /etc/ssh_config (or uncomment and edit the relevant lines):
Host * ForwardX11 yes Protocol 2,1
Finally, type "exit" to get out of the root login.