The reason you did not find anything about Macbeth's ghost is because it does not exist. The only ghost present in Macbeth is the ghost of Banquo, who is arguably a figment of Macbeth's imagination. The difference between this ghost and that in Hamlet is that other people were able to see the Ghost of Hamlet's Father. Also, the Ghost of Hamlet's father spoke directly to Hamlet, telling him what to do, and to whom Hamlet should exact revenge. In Macbeth, the ghost simply served the function of a symbol of Macbeth's overwhelming guilt for murdering Banquo.