It depends. If the viewpoint essay is in a print book, there are two possibilities - either it is written by the author of the book (in which case you cite the book as a whole), or the essay itself has an author listed separate from the book's author or editor (in which case you cite the essay as a work in an anthology.) See OWL at Purdue for examples of each.
If the viewpoint essay is online, for example in one of the library's research databases, you will often find the proper MLA on the same page as the essay. Just make sure the citation they are giving is, in fact, in MLA style. If the citation is not provided, you will need to figure out where the essay was originally published in print, before it was added to the database. See Diana Hacker's site for assistance, or contact a librarian.
