Apple's X11 for Mac OS X is available as an optional installation bundled with Mac OS X. To install it when you first install Mac OS X Tiger (or upgrade an existing installation), you must customize the installation (in the Selection Type phase) and select the X11 checkbox. If you don't install X11 during the Mac OS X installation, you can install it later by inserting the Install Mac OS X DVD, then double-clicking the System folder, followed by the Installation folder, and then the Packages folder. Here you'll find the X11User.pkg package, which you must also double-click to start the installation process.
The installation places the double-clickable X11 application in the /Applications/Utilities folder. If you're going to build X11-based applications, you'll need to install the Xcode Tools, which installs X11SDK by default. To install Xcode Tools along with X11SDK, insert the Mac OS X Install DVD, and double-click the Xcode Tools folder to find XcodeTools.mpkg, which you must double-click to begin the installation process.
If you don't install X11SDK when you install Xcode Tools, you can install it later by once again inserting the Mac OS X Install DVD, double-clicking the Xcode Tools folder, followed by the Packages folder, where you will find the X11SDK.pkg installer. Double-click the X11SDK.pkg installer to begin the installation of X11SDK. Instructions for building X11 applications are included in Chapter 11; this chapter simply focuses on using X11.
The X11User.pkg can be downloaded from http://www.apple.com/macosx/features/x11/download, while Xcode Tools can be downloaded from the Apple Developer Connection located at http://developer.apple.com.