The band U2 has a song called "Do you feel loved?" on their Pop album. The song is a somewhat cutting commentary on what people will do when they don't feel loved. One could argue that Macbeth, despite the respect of his peers, feels that he is not being given his due.
Classical pieces will lend themselves well to the "score" of Macbeth. Certainly, some of Beethoven's ominous compositions such as the famous Fifth Symphony should serve you. Check out Richard Wagner's tumultuous music also, which has much emotional expression in it.
Let me congratulate your teacher on this innovative assignment! Capturing the themes of Macbeth in musical form will greatly enlighten you on the play's textures and rhythms. First, I would warn against any obvious "witch-like" music, just as the costumer should avoid pointed hats, etc. The essence of Act I is the way that human actions (war, promotion in rank, etc.) contrast with the "other-worldly" activities of spell-casting, calling on higher powers, and (self-fulfilling) prophecy. So the music should try to demonstrate this contrast. Second, the other driving force in Act I is ambition vs. fate--to what degree does human will/choice overcome the pre-ordained actions of the universe, if at all? As to what musical vocabulary will accomplish these ends, I suggest dissonance ("Fair is foul, and foul is fair"); close attention to the balance of predictability vs. surprise in melody, orchestation, and key signatures; and an undertone that emphasizes the darkness of the consequences of this contrast as events unfold--something between Carl Orff and Arvo Part?