Where do you add freon to a 1991 Chevy Beretta?

You don't add freon to vehicles. You add R134a refrigerant.

On the other hand, you have a 1991 model, equipped from the factory with R12. If it has not been retrofitted yet, you will need to do so first. It is illegal in most states to expel refrigerant into the air, so you will need to bring your vehicle to a service shop and have them properly evacuate the system before retrofitting it to use R134a. Only then can it be refilled. It may be costly at first, but once done you can buy a recharge kit from most stores and fill it yourself.

If you've already been retrofitted to R134a, attach the recharge kit to the valve under the blue cap on the lower front of the vehicle, between the engine and radiator.