Hi,
this script is in the binary release distribution
http://www.zimbra.com/community/downloads.html
Choose the OS you need and download it.
If you need to install from sources, for Linux, inside the binary there's a file called "readme_source.txt" that give the step's to work with the sources.
If you work with Windows, there's a thread that explain how to install Zimbra for sources.
Developing Zimbra on Eclipse