How do I install Quartz Composer in Mac OS X Lion?
According to Xcode 4.3 release notes, Apple restructured Xcode — what gets installed (and where) has changed.
From Apple's documentation:
Several additional tools are no longer part of the default Xcode installation, they are now downloadable as separate packages. The More Developer Tools menu command (Figure 5) provides a direct jump to developer.apple.com/downloads in Safari where these development tools can be found.
Once you open that and log in with your Apple ID, then download and install "Graphics Tools for Xcode - ...", which includes Quartz Composer.