build.xml usually is an ant build script.
It contains information necessary to build the project to produce the desired output, be it Javadocs, a compiled project, or a JAR file.
I believe Eclipse has ant built-in, so it should be possible to execute the build.xml by choosing “Run As…” and “Ant Build”.
The build.xml file, if it is an ant script, is not used to import the project into an IDE like Eclipse or Netbeans. A build script is used to build the project (or produce some desired output) rather than an mechanism for importing the project into an IDE.