For this step, four trample steams are required (how to build the trample steam)
You need to build one on the table in the first building, all four players are required to obtain one. All four need to be placed on four more gold symbols around the map. Three symbols are located on the rooftop of the second building, and one is located near the elevator just outside the starting room.
Make sure you aim them so they fling zombies at the satellite tower. When enough are killed, the step will be complete.