Just like in a written paper, you need to have a separate list of all the details about about your sources (your Works Cited/References/Bibliography) plus the abbreviated citation to use wherever you've borrowed information (your in-text citation).
For example, a couple of slides discussing the chucking abilities of woodchucks have parenthetical citations:
The last slide of this presentation would be the Works Cited, which provides all the details that didn't appear in the in-text citation (like site name, publication date, and url).
Notice that the formatting in these examples is not exactly conventional MLA: the text is not in Times New Roman size 12 and nothing is double-spaced. Your professor is the final authority, of course, but generally speaking, the citation styles don't have any particular rules for presentations. Remember, slides are a very different presentation medium than a written essay and adjustments must be made for readability and aesthetics.