Lastname, Firstname. "Title of PowerPoint." Course Title, Day Month Year, University, City. Microsoft PowerPoint slides. Title of Website, URL. Accessed Day Month Year.
Horowicz, Kari, and Lara Nicosia. "ARTstor: More than Just Art!" 8 Jan. 2011. MUSE 101-01 Introduction to Museum Studies, 8 Jan. 2011, Rochester Institute of Technology, Rochester. Microsoft PowerPoint slides. myCourses,mycourses.rit.edu/d2l/home/479317. Accessed 12 Dec. 2017.
Intext: (Horowicz and Nicosia slide 3)
Class PowerPoints are not usually retrievable by others outside of the class and should be used sparingly. Microsoft PowerPoint slides should be added to the citation to indicate that you are not citing a standard source type (MLA 52). For PowerPoint slides accessed online, provide the website title and URL, but omit the http:// or https:// (MLA 110). More information can be found here: https://style.mla.org/citing_slides.
This source type is not covered by the MLA Handbook (8th ed.)