Usually recommended tire pressures are marked on the side walls of the tires.
For loader work I recommend the maximum pressure listed. On mine I run about 40-45 PSI in the fronts and 18-20 in the rears.
Yours may vary depending on tire size.
Kubota B8200, then a Kubota L3130 HST, now a Kubota L3400 HST
I've read that, like stated above, you want your fronts at the max. pressure shown on the sidewall: that should be their greatest load-bearing, and adjust the rears so the tread is flat against a flat surface. Somebody, maybe the guy at Everything Attachements (?) demonstrated how they do that.