create the bus and name it whatever, now in the trigger you want it to set by whoever activates it. put this in the activation field of the trigger: Bomb = "Timebomb" camCreate getPos busname now in all the 3 time boxes put 60 ( 1 minute)
now put the units you want to come out behind the building, then put a move waypoint right in front of them and syncronize it with the trigger, then put another waypoint for the units so they come out when the trigger is activated.
then for the units you want to hold fire put a move waypoint set to hold fire, and sync it with the trigger. then put another waypoint set to engage at will so when the trigger is activated, there ready to kill.