I have the same car as you, a 1998 156 2.5 V6. I think you may be getting confused by the manual!
The knob on the left sets the target temperature of the cabin. Turn this fully anticlockwise for now to minimum temperature. The middle knob sets the fan speed. The AUTO allows the car to choose it's own fan speed. e.g if the temperature in the cabin is close to the temp set on the left knob, the fan speed will be slow. If the temp is a lot different, then the speed will be fast. For now, set this knob to 4.
Looking at the left knob again, there is a switch at the bottom of it to activate the aircon. The green light should be on. If it is not on, press the switch. The middle knob also has a switch, which controls whether the air is fully circulated or is mixed with air from outside. For maximum rapid cooling, this should be on. However it should not be left on for long periods, or if you have passengers, as you need some fresh air in the cabin. Turn it on for now.
Vents should be open and I usually set the right hand knob to 'screen' (next to MAX). Turn on the engine.
If you find that with the controls set as described, there is no blower and / or the light on the aircon switch refuses to come on, I suggest hitting the side of the console sharply on the left next to the aircon switch with your hand and you may well find that it all bursts into life. Works for me anyway