OK, so i read a bit more. You need an apex switch box to make this work. You wire in the humidistat to your switch box, and when it kicks on, THEN you can have your apex turn on your fan. If you have the switch box already, this would be the less expensive and more controllable option. I read about it here:
http://www.reefcentral.com/forums/showthread.php?t=1933966 link ($25.09 shipped w/prime)