Not really sure what you mean to do when invoking the build.xml directly? Tried right clicking and selecting Make Module 'helloworld' but I don't think that's what you mean. Or is it just one of the ant targets you can use to invoke it?

In any case, I added a new target to the main build.xml file:

<target name="helloworld">
       <antcall target="sub_build_module">
           <param name="moduleName" value="helloworld" />        </antcall>
   </target>

(Possibly this is what you meant above? Have this target and then running that target from the list of ant scripts)

And, then did you what you mentioned above. Adding the antcall to both build_classes_parallel and build_classes_sequential

<antcall target="helloworld" />

its now appearing in the list of modules.

...Now to figure out how these work :)