You must be able to connect to your database server.  I used hsqldb, so you can run the server like this: 
Code:
java -cp hsqldb-1.7.1.jar org.hsqldb.Server
in maven.xml:
Code:
  <postGoal name="java:compile">
    <taskdef name="schemaexport" classname="net.sf.hibernate.tool.hbm2ddl.SchemaExportTask">
        <classpath>
            <path refid="maven.dependency.classpath"/>
            <pathelement location="${maven.build.dest}"/><!-- our classes must be on the classpath -->
        </classpath>
    </taskdef>
    <mkdir dir="${maven.build.dir}/sql"/>
    <schemaexport quiet="no" text="true" drop="no" delimiter=";"
      properties="${maven.src.dir}/com/foo/bar/myapp/hibernate.properties" 
      output="${maven.build.dir}/sql/schema-export.sql">
      <fileset dir="${maven.xdoclet.hibernatedoclet.destDir}" includes="**/*.hbm.xml"/>
    </schemaexport>
  </postGoal>
Make sure you have a <dependency> for dom4j in project.xml because schemaexport uses it.