What terminals do you have on the thermostat?
What wires are hooked up to which terminals?
What are all the wires you have available?
Green = G controls the blower motor
Red = RC is your 24V power
White = W controls the heat source
Yellow = Y controls the AC
Blue = C is the common wire, if your T-stat is battery powered then it is unused.
If hooking it up like that does not work, disconnect the t-stat, touch red to green and white, if you have heat, the t-stat is bad.
I cannot count on all fingers and toes how many times I have found those hunter brand t-stats to be bad, yes, brand new out of the box.
I highly reccomend to get a honeywell instead.
When you say "on the ac" I assume you mean the package unit.
With a package unit, the AC, the furnace, and the blower are all in one "package".
They may be using the brown wire instead of the white for heat, on your system.
Try what I suggested above but substitute the white wire for the brown.
tell me what you get.
If you get nothing then you will need to check the heat sequencer for low voltage and see if the contact is closing if low voltage is present.
You can also locate the high temp limit switch and see if its tripped.