The bubonic plague did work for the mongols. Killing 1/3 of the population of europe. If ISIS use ebola as a weapon I don't think this will not work for them. Ebola just infected an american nurse in dallas without the terrorist help how much more if the terrorists is added to the equation.
Probably trying to spread ebola wouldn't be a great success. How these infected terrorist can inject their bodily fluids into many people without being noticed? Sneezing and coughing on crowded buses or trains? Yes that could do some harm but I think they would be more effective with a few dozens of traditional hand grenades.