no output thru speakers Mbox

I've just purchased MBox/PT Xpress. I'm all downloaded on MAC/Mavericks. Just trying first to get some output to me speakers. In systems preferences/sound, Output gives me two options - Avid Core AudioDevice -Avid Mbox. Which should I be selecting? When I select either the message is: The selected device has no output controls. Anyone tell me what direction I should be heading to get out put to my speakers. Thanks.