Prelude: Ant-Contrib provides a very well explained wiki page for installation and launch ([http://ant-contrib.sourceforge.net/#install][1]).
Indeed
if you have teh later 1.0b3 version.. from http://sourceforge.net/projects/ant-contrib/ or http://ant-contrib.sourceforge.net
i beleive you have to use this one for ANT 1.6 and later :
<taskdef resource="net/sf/antcontrib/antlib.xml"/>
instead of the antlib.properties file.