There are various configurations of humidifers...in-line and bypass are the two more common as I see it. Steam doesn't seem to be that common on residential units. An in-line one works best in the air duct after the heat exchanger. A bypass unit needs to have the inlet and outlets in different pressure regions of the ductwork so that air is moved across it. If you can put one end after the heat exchanger and the other side before the fan, it would be the most efficient, but as long as there is a pressure difference between the two sides, some air will cross through and pick up moisture. There are lots of places in a typical system where you can tap in to do a bypass, some are more efficient than others. Same is true with an in-line system, but the thing could even be put in the cold air return and still insert moisture...it would work better in the output because the heated air can hold more moisture. In this case, it may not need to run as long and be able to satisfy the demand quicker.
To help keep stratification down, I run my variable speed fan constantly, which also enables the humidifer to work. It's a Trane air handler, and when not calling for heat or a/c, the fan runs on speed 1 of 16, so you don't hear it and the power draw is minimized at the expense of increased wear. It'll die one of these days, but has been running for nearly 15-years now.