When you get to Bald Island don't build the Cauldron yet because the one that is already down will not work or count toward your later task. Instead go to your Market and click the New tab and set down the Cauldron that is in there. Also you can no longer get Z-pumkins so you will probably have to use Zombucks to help finish the building process. You need ten Z-pumpkins to build the Caldron.
I would also suggest not pre building any of the other items in the New section until the task pops up just to make sure that it will count.