IT DEPNDS on the version of OS you are using. Snow leopard will allow you to load Quartz Composer from the disk, YET if you are using Lion, you do NOT get Quartz with XCODE. If you have lion and and want Quartz, the only way I have found to get Quartz is if you have an Apple Developer account.
When you open XCODE in Lion, in the menu bar you will find an option to download extra content. if you click on that option, it will send you to the developers page where you have to log in, then you will be able to download quartz.