This is a great question.
I really doubt you'd be able to convince people to not go to work at the trade center. They'd probably just think you were a loon. Maybe call in a bomb threat that morning to try to get the place evacuated.
I'm not sure how you'd keep the flights grounded either...one maybe - you could go to the airport and cause a ruckus to delay one of the flights from taking off, but that's not going to prevent all the events. Maybe bomb threats are the way to go here as well...
Of course all of this probably lands me in jail, but I guess that's worth it if I save a bunch of lives.
I would definitely try to get access to the trade towers and check for explosives though....