Citing the Online MMY in APA Style

Mathematics Anxiety Rating Scale. Mental Measurements Yearbook, 11th ed. [Electronic version].
    Retrieved January 4, 2002, from Mental Measurements Yearbook database.

McMorris, Robert F. [Reviewer].(1996). Mathematics Anxiety Rating Scale. Mental Measurements
    Yearbook, 14th ed. [Electronic version]. Retrieved January 4, 2002, from Mental Measurements
    Yearbook database.


If you can't find a date, then just leave it out. The publisher isn't necessary in this case. There should be a notation with the edition (11th, 14th, etc.)