Note: formatting shown here is not necessarily exact, because of word processor limitations. Always check a guide such as OWL at Purdue for exact formatting.
Citation from the eNotes study guides is very easy; each page has a dedicated citation button that will create a pop-up window containing the proper citation for that page's information and source. Some information on the site comes from external sources, while other information is original to eNotes. For example, the Themes page of the eNotes study guide for The Rover is cited like this:
"Themes." Drama for Students. Vol. 16. Gale Cengage,
.eNotes.com. 2 Dec, 2012 <http://www.enotes.com/rover/>
You can see that this page is sourced from the textbook Drama for Students, Vol. 16, by Gale Cengage Group, a reference and content provider. In this case, the information originally comes from that textbook, but is referenced and cited from eNotes.com, so the citation shows that the information was accessed from that website, not from the original textbook. If the original textbook is at hand, you could choose to cite it instead of eNotes, even though the information is the same. In that case, the citation would conform to proper MLA format, using publishing information such as author and date on the copyright page of the work; you can format it yourself or use a formatting guide such as the Citation Machine for easier formatting.
However, since eNotes is making that information available with the permission of the authors, it is perfectly acceptable to use the eNotes citation links in your bibliography. Just look for the citation button on every eNotes study guide page, just above the body of text and between the "PDF" and "Facebook" link buttons.