i guess that you can't ever really quote him directly, unless you were writing in greek. so you might have to signify that you are quoting a translation of Plato, by whomever the author is. if the book tells you what dialogue the translation directly comes from, you can signify that as well as citing the book (dialogues Plato) and page number of from it.
it seems, you might have to cite Plato indirectly by referring the reader to "dilogues plato" and it's page number.
just give as much info about the source you are citing from, in your essay, so that the citation is understood, correctly.
if it's a plato translation, it's a plato translation, and you should just need to note the direct source, from which you extracted the translation.
if this doesn't help, then i am at a loss and you might want to see if u can find a single dialogue translation