In most standard dictionaries, no author is given for either the overall dictionary or the individual entries, so no author should be listed in your MLA citations.
Instead, start your Works Cited entry and your MLA in-text citation with the title of the entry you’re citing (i.e. the word that’s being defined), in quotation marks.
If you cite a specialist dictionary that does list an author and/or overall editor, these should be listed in the same way as they would for other citations of books or book chapters.