On the inside of the wheel usually where the spindle & flange meet, through the weld.
Some wheel have em & some don't.
Do it regularly & it is just 1 stroke of the gun and best done with the mower jacked up so there is no weight on the wheels.
How is the grease going to penetrate the bearing when unit is sealed for maintenance free operation. I understand hitting the grease zerks in front end.
It doesn't.
Like grease in a blade spindle housing with doubled sealed bearings at both ends, it fills the void preventing water getting into there.
It stops the axel from rusting so you can get the wheel off when you need to fix a flat.
For those of you that are following the thread. I received a note directly from Hustler stating that the axles should be greased when the oil is changed. I did grease mine at about 10 hours. They both took an awful lot of grease before they were full.