I won’t go though explaining how to create a build.xml file, the official manual, which is really good, does it better than I would.

The error in question occurs because the java executable, after reading the Manifest file from the inside of the JAR, doesn’t know what is the entry point of the program; i.e. the class that contains the ugly public static void main(String[] args) method.

To fix the error, that class’ full name needs to be specified in the build.xml file.
For example:

<jar destfile="${jar}" basedir="${build}">
    <manifest>
      <attribute name="Main-Class" value="my.awesome.package.Main" />
    </manifest>
</jar>