Once you get the Pack-A-Punch area opened up, have one player lead your crawlers to somewhere near Stamin-Up and get all the players gathered at the launch pad in the Pack-A-Punch area.
You will see a clock on the wall that reads 12:00, and you must stand on the pressure plate in front of the clock. The plate is quite large so don't worry about falling off, but it is preferred to stand still or stay in a generally close area. You must stand on the plate for 2 minutes, but can shoot or throw grenades in case the crawlers get close and must be killed.
After the two minutes is up, there will be a nuke that goes off and ends the round. At this point, Node 3 of the Easter Egg should be completed. Back at the metal chunk near Stamin-Up, 3 lights should appear as lit up on the metal.