Aprilaire Model 500M and if the humidistat is the Manual Humidifer Control, Yes it came with the unit.
ok here you go.
If this humidistat is digital this is not the same so if it is let me know.
one wire from the solenoid goes to one wire on the humdistat
Other wire from solenoid goes to one side of the supplied 24v trasformer
Other wire from humidistat goes to other side of the 24v transformer
Then the high voltage black and white leads of the supplied humidistat
white wire goes to one of the unused Neutral spade terminals ont he control board
Black wire goes to the "HUM" spade terminal just to the left of this bank of nuetral terminals.
The Manual Humidifier Control didn't come with high voltage black and white leads There is only 2 terminals That I wired with Thermostat wire.