One thing you could do is change the situation from kings and princes to a corporation or some other modern day situation where people are competing for power. Duncan could be the CEO, Malcolm his son, and Macbeth could be the guy who just oversaw a big merger or some grand business deal. The witches could be professors of economics, sociology and human behavior, and philosophy (Nietzsche or Machiavelli would work well). Maybe Macbeth goes to them for advice since they are all advisers on the dissertation he never finished.
Or, maybe all the characters are homeless and live in Central Park. And Macbeth, after years of harsh living, has gone a bit crazy, and three pigeons, whom he perceives to be future-telling witches tell him what to do in order to take over the park. This scenario is a bit more strange, but just to show how you could go any number of directions with this. Have fun.
I'd encourage anyone to be original and creative, but if you get stuck or need an example of a modern version, check out the movie Scotland, PA.
To create a modern day version of the play 'Macbeth' by William Shakespeare it would be a good idea to consider what modern day audiences want and that is often novelty. Many audiences crave something different - particularly the young - whilst older older audiences might like the traditional version. Some ideas would be to create a modern urban play where the characters wear suits and carry laptops and ipods. Their idea of a 'king' could be their top=producing employee or boss instead of royalty. You could do a minimalist play, mostly dark, with no props and sparse costume where the 'witches' are truly invisible, perhaps appearing only as pools of light. The actors would have to be very accomplished to convey the drama and hold the audience. Contemporary Dance could be used to emphasise the action.