Reading data from firefox install.rdf with Javascript

I'm very new to RDF and I don't understand how I can determine the value (called literal??) associated
to an attribute.

Suppose I have this RDF

<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
     xmlns:em="http://www.mozilla.org/2004/em-rdf#">

  <Description about="urn:mozilla:install-manifest">
    <em:id>{11111111-2222-3333-4444-555555555555}</em:id>
    <em:name>Extension name</em:name>
    ...
    ...
    ...
    <em:homepageURL>http://asimpleurl.com</em:homepageURL>

How can I obtain the value http://asimpleurl.com??

I tried code similar to this below but doesn't work.

var rdfs = Components.classes["@mozilla.org/rdf/rdf-service;1"]
                   .getService(Components.interfaces.nsIRDFService);

var ds = rdfs.GetDataSourceBlocking("file:///tmp/install.rdf");
var extension = rdfs.GetResource("11111111-2222-3333-4444-555555555555");

var homepageArc = rdfs.GetResource("http://www.mozilla.org/2004/em-rdf#homepageURL");
var homepage = ds.GetTarget(extension, homepageArc, true);

may use help me to understand how to handle RDFs?
many thanks,

davide