Electrical – Aprilaire 56 Humidistat not getting power

It's likely functioning properly. A setting of 6, probably correlates to 40% relative humidity. According to the Owners Manual

Your Aprilaire Automatic Humidifier, is a high precision system that will accurately maintain the relative humidity in your home to a maximum of 45% RH and a minimum of 10% RH.

If the home is at a higher relative humidity, the unit should not come on.

There's two ways to wire this up. The first method is to take advantage of the HUM terminal on the furnace control board. This terminal will supply 120 volts when the furnace is running in heat mode, so you'll have to use an additional transformer.

You'll hook the primary side of the transformer to the HUM, and Neutral terminals in the furnace. Then you'll connect the 24VAC terminals of the humidistat, to the secondary of the transformer.

The other method, is to use the furnaces transformer to provide power to the humidistat. However, you'll have to make sure the transformer is large enough to supply the load.

With this setup, you'll connect the 24VAC terminals of the humidistat to the C and W terminals on the furnace control board. That way when the thermostat calls for heat, the humidistat will be powered by the furnaces transformer.