How would you explain the essay prompt "power corrupts but absolute power corrupts absolutely" in response to Orwell's Animal Farm and Shakespeare's Macbeth?

Great question. I think the best approach is to first consider the structure of your essay. You can have at the back of your mind an idea of how this idea applies to each story—who in Animal Farm is corrupted by power? Who is corrupted in Macbeth? What is the difference between power and "absolute" power, and where is the turning point at which the corrupted parties become corrupted "absolutely," if you feel they do? But remember, start with your introduction and your thesis statement.
In your introduction/thesis statement, you should lay out to the reader what your point of view is on this quotation and how it relates to the two texts. You can begin by identifying the source of the statement: Lord Acton, whose point was that a person's morality diminishes as his or her power increases. The most straightforward thesis statement would be to argue that this is shown to be true in both texts and then have the rest of your essay demonstrate how. However, you can also make counter-arguments, such as the fact that Duncan in Macbeth does not appear to have been corrupted by his power, and that Macbeth seems to have been corrupted not by power alone, but by pressure from those around him (e.g., Lady Macbeth and the witches).
You might want to argue one text first, then the other. This will help you organize your thoughts on the subject. So, if we start with Animal Farm, we can begin by arguing that Jones, who had absolute power over the animals, was corrupt and that Napoleon and the other pigs were, when powerless, very idealistic. However, over the course of the book, we see a change in the pigs as their power increases. Eventually, they begin to do the same things as the humans they once hated; at the end of the novel, they have symbolically become the corrupt overlords as it is "impossible to tell which was which" between the pigs and the men they are playing cards with.
In Macbeth, the person corrupted and destroyed by power is the title character. His ambition causes him to kill the sitting king, who has always been good to him, and eventually leads him to have his best friend killed too. Here, however, you can make some further arguments against the point. Was Macbeth corrupted by power itself, or by the desire for power? Or was it the desire to please his wife? Or fear of what the supernatural told him? How far are the witches, as well as Lady Macbeth, to blame?
At the end, you can draw together the points you have made and conclude as to how far this statement applies to the two texts.