<target name=”retrieve” depends=”proxy”>
<sf:retrieve
username=”${sf.sandboxusername}”
password=”${sf.sandboxpassword}”
serverurl=”${sf.sandboxurl}”
retrieveTarget=”Retrieve”
unpackaged=”package.xml”/>
</target>

Sample package.xml:

<?xml version=”1.0″ encoding=”UTF-8″?>
<Package xmlns=”http://soap.sforce.com/2006/04/metadata”>
    <types>
<members>*</members>
<name>CustomObject</name>
</types>
<version>30.0</version>
</Package>


Cheers!!!