Q. How do I cite a line from Macbeth in MLA style?

Since you are citing from a published copy of the play, you'd cite it basically like a book. This is an example from MLA Style Center:

 Miller, Arthur. Death of a Salesman. Bloomsbury Publishing, 2015.

Your in text citation is always derived from your works cited entry, so an in text example would be: 

"your quote" (Miller 45)