If you are citing an article from a website, include the web address of the article, and then write the word “accessed”, along with the date on which you accessed the website (written in the format of “month day, year”) in parentheses.
Smith, John. “Studies in pop rocks and Coke.” Weird Science 12 (2009): 78-93. http://www.weirdscience.org/articles/id=1212 (accessed February 21, 2009).
For an article found in a database, write the database URL instead of the website URL and make sure to cite the date on which you accessed the article.
Smith, John. “Studies in pop rocks and Coke.” Weird Science 12 (2009): 78-93. http://www.lexisnexis.com (accessed February 21, 2009).