The WebObjects543.dmg file will install is the last WebObjects installer package available from Apple (dated September 2008). It installs the WebObjects frameworks and support files . It requires as four sub-packages ("WebObjectsDevelopment", "WebObjectsDocumentation", "WebObjectsExamples" and "WebObjectsRuntime"), into the appropriate directories on a Mac OS X system for incorporation into Apple's Xcode 3.1 . It is recommended that you follow the Installing WebObjects 5.4 on Snow Leopard instructions if you no longer have Xcode 3.1IDE.
The "WebObjectsRuntime" sub-package is the critical because it contains all the Java libraries (in "Framework" and jar packaging) required to develop and deploy a WebObjects application. Since such applications are now developed using the multi-platform Eclipse IDE, and not Xcode, and are deployed on as many, or more, platforms, the "WebObjectsRuntime" package is also now available for multi-platform use from the community that brings you the page you are reading now!
The currently recommended WebObjects installation procedure for all platforms is at Step 2.2 on the Project Wonder Installation page.