The Aprilaire 760 mounts on the "supply" or plenum side, not the "return drop" or "cold air" side. It's spec'd for operation at a 120 degree plenum temperature. Using hot water for the supply makes it a bit more efficient as well.
Be careful! don't get into the airconditioning coil when mounting the humidifier. Typically, for an upflow furnace with air conditioning, you'll see a couple of pipes running out of the plenum a few inches to about a foot above the top of the furnace. These go to the airconditioning coil. Mount the humidifier above the coil so everything can still be accessed for service.
Since the 760 is powered by a 110-120 volt supply you'll use the induction sensor in series with the "A/A" terminals on the humidistat to get the unit to only run when the humidistat calls for more humidity AND the furnace blower is on.
Note: Check your local codes. Most plumbing, heating, and AC shops will answer your questions.
The installation instructions suggest tapping the 110-120 volt supply before it enters the furnace. Check your breaker box. If the furnace is on a 15 amp circuit (most are), and you have a switch box mounted on your furnace; then, USUALLY, you can meet code if you use the existing switch at the furnace so both the furnace and the humidifier are "off" when the switch is off. But not always. Some places require a seperate switch or even a seperate circuit along with a seperate switch.