PowerPoint from a Lecture
If you saw the PowerPoint at lecture or conference, you should cite the lecture. Include the presenter, presentation title, name of conference, location venue and city, date of the lecture, and kind of lecture (or just write "Lecture.").
For example: Smith, Jane. "Emily Dickinson." Poetry Convention. Hilton Hotel, Charlottesville, VA. 12 April 2013. Keynote address.
PowerPoint on Computer
If you simply have the PowerPoint on your computer, cite the presentation using the digital file format. Include as much information as you have, such as the author's name, the name of the presentation, the date the file was created and the medium.
For example: Smith, Jane. "Emily Dickinson." May 2011. PowerPoint presentation.