You can replace the designer version in $jbpm_installer/build.properties:
designer.version=1.0.0.053-jboss
designer.url=http://sourceforge.net/projects/jbpm/files/designer/designer-1.0.0.053/
and run ant clean.demo and ant install.demo for it to get downloaded and copied to the right location.
Also make sure you clear your browser cache before using it again.
If you also want to use the workitem configuration editor shown in that video, you would also now need to clone Guvnor master from https://github.com/droolsjbpm/guvnor and build it with
-Dfull -DskipTests
then use the
guvnor-distribution-wars-5.3.0-SNAPSHOT-jboss-as-5.1.war
Hope this helps.