Q. How do I cite a play that I've seen in MLA style?

According to The MLA Handbook Online 9th Edition, this is how you cite a play performance:  

Last Name, First Name (Play Author).  Name of Play.  Directed by (name of director), Name of Theatre Company, DD MM YY (date seen), Name of Theatre, City of Theatre.

Example:

Shaw, George Bernard.  Heartbreak House.  Directed by Robin Lefevre, Roundabout Theatre Company, 11 Oct. 2006, American Airlines Theatre, New York City.