Hi Diogo, if you are using the Eclipse editor in jBPM or the web designer, check out this blog and video which I think will help you: http://blog.athico.com/2011/09/jbpm5-tooling-support-for-service-nodes.html.
If you are using the new BPMN2 Eclipse editor, then I don't think there service nodes are supported as of yet...but I could be wrong.
Hope this helps.
Did you solve the problem? I am facing the same issue where the service task is not found anywhere in the palette in the BPMN2 editor