You must include the "medium" of publication for all MLA citations. The most commonly used mediums are 'Print' and 'Web', but there are many others. Refer to the complete list of MLA citation examples for placement. Here are a few examples:
MacDonald, Richard G. Biochemistry. New York: McGraw, 2007. Print. (book)
Web
Potter, Michelle. "Zora Neale Hurston: A Literary Life." LiteraryTraveler.com n.d. Web. 22 Apr. 2011. (Website)
CD-ROM
"The Chemistry of Air Pollution." Magill's Survey of Science. 1998 ed. Pasadena: Salem, 1988. CD-ROM. (CD-ROM)
Microsoft Word file
Cortez, Juan. "Border Crossing in Chicano Narrative." 2007. Microsoft Word file. (Word document on your computer)
eBook
Smith, Diane. "Re: Hurston." Message to David S. Nevins. 21 Aug. 2009. E-mail. (email to a colleague)