Considering that this play is all about the potential corruption of people in power, I would say that the quote, "The law makers are the law breakers" would refer to the fact that Duke Vincentio is trying to catch those people of Vienna who are using their power for their own gain, rather than for the good of the people of Vienna. The people in power, "the law makers," have become those who simply do what they want to satisfy their own greed, lust, etc., "the law breakers."
Unfortunately, in doing a quick re-read of the play, I cannot find that exact quote. Can you let us know which act and scene number it can be found in? That would better able us to give you a more precise answer next time.
Check the links below for more information about this play - Good luck! :)