Insert your OS X Install Disk #1.

A finder window will open that will show you all the installers and folders on the disk.

You have to double-click on the Optional Installs package.

PS: However, due to the organization of the icons in the finder window, you must scroll down to find the Optional Install icon.

Mac OS X Tiger

go to http://static.macosforge.org/xquartz/downloads/Tiger/, download the source file, install it on the computer.