Sample build.xml:


  <target name=”retrieve” depends=”proxy”>
  <sf:retrieve
  username=”${sf.sandboxusername}”
  password=”${sf.sandboxpassword}”
  serverurl=”${sf.sandboxurl}”
  retrieveTarget=”Retrieve”
  packageNames=”Package_Name“/>
 </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!!!