Embedded OSGi: accessing dynamically loaded bundle components

I'm getting a little lost when dealing with embedding OSGi into an application/library, loading a bundle at runtime and then using that bundle in the host application/library.

This is code to load the bundle from my host:

// framework is the embedded OSGi framework
Bundle b = framework.getBundleContext().installBundle( "file:/project_dir/bundle/MyBundle.jar" );
b.start();

And the bundle consists of something really simple:

public class Activator implements BundleActivator
{
  @Override
  public void start( BundleContext context )
  {
  }

  @Override
  public void stop( BundleContext context )
  {
  }

  public String shout()
  {
    return "let it all out";
  }
}

How would I access the shout() method from the host application?