It's a very good tutorial how to Install X11 on Tiger step by step.
First Insert your install disk.
1.Look for "Optional Installs" in the installation
2.Run the program:
4.We obtained this window with small triangles in fact sub-menus must be deployed:
5.Select X11 (note: the picture there is 0 bytes because the installation is already done with me):
6.After leaving the setup and put away your disc carefully, find X11 in Applications/Utilities.
Notes: Do not ask how to install X11 on Tiger (wait Leopard).