Its a seperate bottle you buy and add at any stage of re-filling the radiator.
Most additives on the market for cars, are bunk, but RWW is one that actually works.
As for brand it doesnt really matter as long as you get one thats silicate free are the best.
Orange is fine too but you gotto flush the system out well first if you are switching from green.
Nissan Green and Toyota red are good ones as well.
Fred..
