This is a good question and a great example of how those little technicalities can make creating a citation page with MLA format so tricky. The standard rule for authors who have a prefix with their last name like le, du, di, del, des, is to alphabetize the name according to the prefix. If the last name is French and begins with de, and is only one syllable, then you list the author's name with the prefix first, like 'De Blanc, Raoul.' If the author's last name has more than one syllable, then you list the main part of the last name first, like 'Maupassant, Guy de.'
Now in the case of Marie de France-- the above mentioned rules go out the window. 'De France' is really not her last name. It is the thirteenth century, and she was actually from France. In the case of an author name like this, you would list her full name, beginning with Marie. Keep in mind that if your work has been translated, you should include the translator in the citation.
I used my own copy to make an example:
Marie de France. The Lais of Marie de France.Trans. Glyn S. Burgess. New York: Penguin, 1999.