PrimeFaces CSS look'n'feel missing和JS " Uncaught Reference Error: PrimeFaces is not defined "

I am trying to use Picklist from Primefaces. When the page is rendered, the javascript engine in Chrome cannot find Primefaces object. I get the following error 'Uncaught ReferenceError: PrimeFaces is not defined'. Am I missing to include any resource (js) in my .xhtml file? Please advise. Thanks.I am trying to use Picklist from Primefaces. Wh