you should check the manual if the 6 takes straight coolant or a 50-50 mixture of coolant and water (some auto places sell the 50-50 mix so u dont have to do it yourself), then pop the hood and fill it up to the max line when the car is off. if you add it when the car is on, the coolant level wont be accurate because some of it will have been sucked into the cooling system as the engine runs.
since you said your car isnt overheating, dont add straight water, thats pretty much for emergencies. adding water will rust out your radiator, and plus, the boiling temperatures of water and coolant are a big difference, water will be hotter on your engine
dont wait to check up on this cuz last thing you want is to overheat the engine, thats going to do some damage, i think the 6 has an aluminum engine tho dont quote me on that, all i know is my brother warned me bout keeping the 3 engine cooled cuz its pretty light and delicate.
and if the coolant level is fine, check your hoses, or maybe the gauge needs to be fixed.... good luck