The last page of the UML Documentation wizard contains an option to generate an Ant build file based on the JavaDoc options that you have specified to the wizard.
ant javadoc
Otherwise, you need to specify which buildfile to use:
ant -buildfile <name of xml file> javadoc
This generates the JavaDoc documentation of your project.
Go to the Navigator view and locate the xml file.
Right click on the xml file and choose Run Ant.
The Ant script only has one target. So click on Finish.
The Log Console view should appear showing the output/messages from Ant.