Stan you have received some very good advice but everyone has missed the obvious. The manufacturer does not include directions because the average homeowner should not be attempting this. Why do you think HVAC companies have to be licensed and the counties require permits for this install? For your protection is part of this answer.
These units installed improperly can cause a danger to you and your family. Not to mention a waste of your money. I am not trying to say that this job is difficult, it is far from it.
If you have a newer furnace, it probably has a spot on the circuit board to wire the humidifier to. That way the humidifier is only powered when the Blower motor is runing. It installs into the plenum above the A-coil, use a saddle valve to attach to the cold water supply and install the humidistat into the return air duct or into the living space near the thermostat.
If your furnace does not have a terminal for the humdifier then you have to have a power sensor and I will not threaten your safety trying to explain that install to you. But it still isn't rocket science.
I think if the mfg's included some type of installation instructions they couldn't afford the legal expenses of all the lawsuits filed y "handy" homeowners that thought they understood wiring electrical appliances.