This is a great question. I would say that there are several considerations here, because enotes is such a huge website with lots of information.
First, when it comes to the study guides, right under the title, there is citation button (right next to the PDF button). It is fairly easy to miss, but it is there. When you click on it, you will see an electronic citation that you can use. For example, if you are using the guide on Lord of the Flies, and you click on the button, you will see:
"Introduction." Novels for Students. Vol. 2. Gale Cengage, .eNotes.com. 6 Mar, 2012 <http://www.enotes.com/lord-of-the-flies/>
Second, since most of the anwers are short, what you can do is to look up the sources on the bottom of the page and hit the citation button again to get your citation information. I will link The Lord of the Flies, so that you can see an example.