Generally, you should identify a law in an APA reference entry by its location in the United States Code (U.S.C.).
But if the law is either spread across various sections of the code or not featured in the code at all, include the public law number in addition to information on the source you accessed the law in, e.g.:
Lilly Ledbetter Fair Pay Act of 2009, Pub. L. No. 111-2, 123 Stat. 5 (2009).