Problema com view helpers - nao achado

Eu to seguindo este tutorial aeki

http://aaronsaray.com/blog/2011/05/20/themetemplate-system-in-zend-framework/

mas esta dando o seguinte erro:

Message: Plugin by name 'Theme' was not found in the registry; used paths: Zend_View_Helper_: Zend/View/Helper/;C:\Zend\Apache2\htdocs\zfteste\application/views/simple\helpers/

como resolver?