Comments for Comfort Made in the USA!
If it is on the supply side, it is most likely installed correctly since either a power or bypass humidifier would be installed there.
A bypass humidifier gets its air movement through the Water Panel® by the differential pressure between the supply (hot air) plenum and the return (cold air) duct. The supply plenum is under positive pressure and the return duct is under negative pressure.
One of the advantages of a bypass humidifier is that it can be installed either on the supply plenum or on the return duct with equal effectiveness. No matter which duct the unit is mounted on, the airflow always goes from the supply to the return through the humidifier.
If your unit is a power humidifier, it would be correct to be mounted on the supply side.
Hope that helps.