Add "C" wire to NEST
Sometimes air handlers/furnaces have lift off covers that allow you to access the wires, but often you will have to remove some screws or cut some duct tape (if the install was messy) to remove the proper panel.  Pictures help here, but you want to look for any covers or panels that will slide or lift out of place.  

If you find one, open it and see whats inside.  If it only provides access to the blower or the heater portion of the unit and not the control board, you will most likely have to remove a few screws to remove another panel (panel can be large, like the whole half of the unit).

Again, this is general advice, but I think that many people might not think they need to start pulling out sheet metal screws and expect a pop-open panel.  If you're not sure, post some pictures.  I wouldn't want you taking apart the wrong thing!