To package an ABL web application ANT project in release 12, you need to update the existing build.xml file. Add the following code in build.xml:

<pathelement location="${dlc.java.dir}/ext/jettison-1.2.jar"/>
<pathelement location="${dlc.java.dir}/ext/commons-logging-1.2.jar"/>
<pathelement location="${dlc.java.dir}/ext/xmlschema-core-2.2.1.jar"/>

For more details, see Package an ABL WebApp ANT project in Progress Developer Studio for OpenEdge Online Help.