lol sorry to spoil the fun
your probably doing this to see if you can do it yourself, but incase you just want a humidifier that turns on when its too humid, i believe they exist, and i have 2 in my basement, but i have to agree, i'd much rather build that circuit myself...
also, if you want to do this, i also suggest a constantly on "dehumidifier" and you connect a relay to the arduino, and then you just turn it on and off like a switch using the arduino and your humidity sensor.