First, insert your Mac OS X Install Disc
Double-click "Optional Installs.pkg"
You'll be greeted with the Installer.
Click Continue.
Click Continue.
Click Continue.
Click Agree.
Select the disk you'd like to install X11 on and click Continue
Select the checkbox next to X11 within the Applications group and click "Install" or "Upgrade".
Type your admin password. When X11 has finished installing, the installer will display "The software was successfully installed".