When is a comma used before et al. in MLA style?

In MLA style, a comma is generally only used before et al. in the “Author” slot of works-cited-list entries when the author’s first and last names are reversed:

Burdick, Anne, et al. Digital_Humanities. MIT P, 2012. 

The comma tells your reader that the name Anne is out of normal position and that the abbreviation attaches to the full name, not just to Anne.

In contrast, in an entry starting with a name that is not reversed—for example, a Chinese, Japanese, or Korean name—no comma is needed:

Liu Chang et al. “Cong Changchungong dao Zhongcuigong.” Zijincheng, vol. 175, 2009, pp. 14-23.

This is because the abbreviation stands for “and others,” and you should use it just as you would the phrase: 

The article was written by Liu Chang and others.

For the same reason, a comma is also not used before et al. in the “Other contributors” slot: 

Dewar, James A., and Peng Hwa Ang. “The Cultural Consequences of Printing and the Internet.” Agent of Change: Print Culture Studies after Elizabeth L. Eisenstein, edited by Sabrina Alcorn Baron et al., U of Massachusetts P / Center for the Book, Library of Congress, 2007, pp. 365-77.  

In other words, “Sabrina Alcorn Baron et al.” is the equivalent of “Sabrina Alcorn Baron and others.” Since the phrase only contains two items—“Sabrina Alcorn Baron” and “others”—no comma appears before “and.”