X11 is installed with the Tiger DVD as an option. Click through the install menus and when you get to the 'modify installation' option you choose to install X11.
> I am trying to install Matlab R14 service pack 3 on the latest > version of OS X Tiger (10.4.3). When I run install, Matlab claims > not to find X11. Yet I believe from Apple's page that on Tiger, X11 > comes ready-installed. Just in case, I reinstalled Xcode tools > (same > problem) and even tried to download X11 from apple website (it > wouldn't let me install, saying that I had a newer version already > installed).