Don't add Javadoc to the Library Manager or Platform Manager in Netbeans. That's not what they're for. I don't what Library Manager or Platform Manager mean in JCreator, but Netbeans isn't JCreator.
Netbeans handles javadoc by default when you create a project and allow Netbeans to create the build.xml ant script. To add javadoc capability to a free form project (existing ant script), do the following. In the project.xml file in the nbproject folder, look for where the "build" action is. Add a new action like the following:
<action name="javadoc">
<script>nbproject/ide-javadoc.xml</script>
<target>ide-javadoc</target>
</action>
Then create the ide-javadoc.xml file in the nbproject folder. It could look like this:
<?xml version="1.0" encoding="UTF-8"?>
<project name="Custom-IDE" basedir=".">
<target name="ide-javadoc">
<javadoc destdir="../javadoc">
<fileset dir="../src" includes="**/*.java"/>
</javadoc>
</target>
</project>
Finally, within the IDE, check the project properties, build and run. Make sure the generate javadoc task is associated with the ide-javadoc ant task.
After all that, the Netbeans generate javadocs command will be available just as it is when Netbeans creates the Ant script.