First, let's be clear about what a parody is. Entoes has a great definition:
A parody (also called send-up or spoof). . . is a work created to mock, comment on, or poke fun at an original work, its subject, author, style, or some other target, by means of humorous, satiric or ironic imitation.
So, there are a number of ways in which you could approach parodying Shakespeare's second act of Macbeth. You could send-up the tense atmosphere created at the opening of the Act and during the exchange between Macbeth and Lady Macbeth in scene ii. In this sort of parody, you would want to look at all the elements that Shakespeare uses to create suspense and a creepy atmosphere (sounds, characters not knowing who's onstage -- "Who's there?", etc.). You should then work to exaggerate the instances of these events so the sense of suspense and the tools used to create it become funny rather than suspenseful.
You can also create a parody of the language (the use of verse) in the play or of the characters. Really, you simply need to pick something about Act II to focus on and work to exaggerate it to the point that it has an "over the top" humorous tone.
This Act is a bit odd to choose to parody in total, since the first and second parts of the Act have quite different tones. The overriding point of focus of the Act, however, is the murder of Duncan, so some exaggeration of this could work well. (For example, you could make it way TOO obvious that Macbeth has murdered Duncan, but no one sees it.)
For more on parody and Act II, please follow the links below.