According to the sixth edition of the American Psychological Association style guide, a slideshow is cited on your reference list only if it can be retrieved--that is, if you can provide your reader with the information they need to find it themselves. If you don't have this information, you instead cite slideshows as though they were information you personally heard.
Slideshows On Your Reference List
You can only include a slideshow on your reference list if it can be found online -- for instance, if your professor posted the slides on the course website. To cite a retrievable slideshow, include the name of the author, the slideshow date, title and URL: Author Lastname, First Initial(s). (Year). Title of slideshow: Subtitle here [Type of document]. Retrieved from URL.
For example: Robinson, P. (2009). Historical review of microscopic imaging [PowerPoint slides]. Retrieved from http://www.cyto.purdue.edu/flowcyt/educate/pptslide.htm.
In-text citations are used in APA style each time you quote or reference a source in your paper's body. If you were able to include your source on your reference list, include the author name and date in your in-text citation.
For example: The etymology of "telescope" has its origins in Greek, and means "to see far" (Robinson, 2009).
If you cannot provide a retrieval address for the slides online, the slideshow is referenced as a personal communication from the person who presented the slideshow. Personal communications are cited in-text only, and include the name of the author and the full date of the communication.
For example: The Fourth Wave's proponents emphasize the importance of self-determination in social movements (B. Conuel, personal communication, May 5, 2014).