Using HttpService provided by Felix Jetty bundle

I'm new in using Felix implementation of OSGi and I'm trying to start the Jetty bundle and register a simple servlet, but I have a problem in getting the Service Reference of HttpService. In details, I made the following steps:
1) Get Felix framework from the proper site
2) Get the Jetty bundle (org.apache.felix.http.jetty) from the same site and insert it in the /bundle folder (the auto-deploy folder)
3) Write my little training bundle made by Activator+Little servlet+Manifest file
4) Create a jar of my bundle
5) Put the jar into /bundle folder (the same where Jetty bundle is put in)
6) In the config.properties file write the property org.apache.felix.http.enable=true
7) Start the felix framework typing "sudo java -jar /bin/felix.jar" from the install folder of felix

In this way it seems that Jetty starts on localhost:8080 (I see 404 Error into the browser), but I have no reference for HttpService. In details, in the activator I have this line:

ServiceReference<?> ref=context.getServiceReference(HttpService.class.getName());

but ref is always null. The same happens if I start Jetty and then my little bundle. What am I doing wrong?