How do I specify a different default installation directory for Unix and Windows?

Let's say you want to use c:\myapp-4.0 for Windows and /usr/local/myapp-4.0 for Unix as the default installation directory. In InstallBuilder, go to "Customization" screen and then to the "Installer" tab. Specify /usr/local/${product_shortname}-${product_version} as Installation Directory and Save the project.xml file. Now edit your project.xml file with your favorite text editor and replace the line <preInstallationActionList/> with this code:

<preInstallationActionList>
  <setInstallerVariable>
    <name>installdir</name>
      <value>${env(SYSTEMDRIVE)}/${product_shortname}- ${product_version}</value>
      <ruleList>
        <platformTest>
          <type>windows</type>
        </platformTest>
      </ruleList>
  </setInstallerVariable>
</preInstallationActionList>

Remember that if you choose /usr/local as the destination for your application you have to require that the installation is performed as the root user. ${product_shortname} and ${product_version} are installer variables. You can find the complete list of the default installer variables available and their meaning in the product documentation included with InstallBuilder.