When you cite an experiment from OPL, you should use the style set forth in the sixth edition of the Publication Manual of the American Psychological Association (2010). The citation includes the name of the experiment followed by the letters "n.d.", which stand for no date because the OPL exercises do not list a specific date of publication or creation.
The first example below is the general format. The second example presents the format you would use for the Mental Rotation experiment. In the specific citation, you start with the name of the exercise (omitting the pointy brackets), the indicator of no specific date, the mention of the Online Psychology Laboratory, and the URL from which you retrieved the experiment.
For Mental Rotation:
Mental Rotation (n.d.) Online Psychological Laboratory. Retrieved from
In a more general form, it would be:
< Experiment Name > (n.d.). Online Psychological Laboratory. Retrieved from
http://opl.apa.org/Experiments/< Experiment Name >