How likely am I to be infected by anthrax during a terrorist plot?

Unlikely. Unless you were directly exposed, inhaling the spores of anthrax at the time they were suspended in the air, your risk is minimal. Infected persons do not spred the anthrax to others like you would see with a flu. See www.Bt.Cdc.Gov/agent/anthrax.
Not likely. Terrorists can more easily handle explosives than biologics. Biologics require sophisticated handling techniques, are likely to escape the minimal containment that a terrorist might use and if the wind is blowing the wrong way, might cause more harm to the terrorists own people than to his intended target. Anthrax falls under this cautionary note. You have better things to worry about.