Fortune is another word for chance. The ancient view of human affairs frequently referred to the "Wheel of Fortune," according to which human life was something of a lottery. One could rise to the top of the wheel and enjoy the benefits of superiority, but only for a while. With an unpredictable swing up or down, one could equally easily crash to the base of the wheel.
Fate, on the other hand, is fixed. In a fatalistic universe, the length and outcome of one's life (destiny) is predetermined by external forces. In Macbeth, the Witches represent this influence. The play makes an important distinction: Fate may dictate what will be, but how that destiny comes about is a matter of chance and (in a Christian world such as Macbeth's) of man's own choice or free will.
Although Macbeth is told he will become king, he is not told how to achieve the position of king: that much is up to him. We cannot blame him for becoming king (it is his Destiny), but we can blame him for the way in which he chooses to get there (by his own free will).