I am wondering where I should have my humidifier in my home. Should it be by a cold air return or farther away, or by the thermostat? Does it even matter as long as I keep it running?

Near a cold air return is good so you can get the humidity moved all around the house. The thermostat won't sense humidity and there is no reason at all to place it by the 'stat.

I have just installed an Aprilaire 700 Humidifier. I seems to work in the "test or reset mode" but that's all. I found the 24 volt transformer, as well as all the other thermostat wires. Some of the thermostat wires have voltage and some don't . This is depending on what the gas furnace is doing when it is running or starting to run. Anyhow I used the 24 volt power straight off the transformer so it's hot all the time. The instructions refer to the FURNACE ACCESSORY TERMINALS not sure where this is at. The transformer has terminals on it. 120volt on one side and 24volt on the other. Can somebody please help?? Thank you, Larry