First of all you need to declare the task in your build file:
<taskdef
classpathref="some.classpath.id"
classname="com.geekologue.md4j.tools.ant.HibernateMappingProcessorTask"
name="md4j"
/>
Alternatively ommiting the classpathref attribute (or equal) should work just fine
if you place the MD4J JAR in $ANT_HOME/lib (%ANT_HOME%\lib on windows).
Using the task after declaring it is rather straighforward. A typical example is:
<md4j basedir="${mappings.dir}" destdir="${build.dir}/src"
excludes="**/named-queries.hbm.xml,**/*hibernate*.xml" force="true"
packageName="${app.package.pojos}">
<contextParam name="ejbdocletViewMethod" value="both"/>
<contextParam name="remoteEjbViewMethod" value="false"/>
<xmlcatalog>
<dtd publicId="-//Hibernate/Hibernate Mapping DTD 3.0//EN"
location="${dtds.dir}/hibernate-mapping-3.0.dtd"/>
</xmlcatalog>
</md4j>