from the MSDN website:
Orca Documentation
Orca is a GUI .msi file editor that is provided with the Windows Installer SDK. It provides full access to the Windows Installer database tables.
A set of Help files are provided with Orca. To obtain the orca.exe and Orca documentation you must download and install the Windows Installer SDK, which is provided as an Orca.msi file. After you install the
Platform SDK Update, double click the Orca.msi file to install Orca.
Note It is not possible to download Orca separately.