VW will tell you that you must use their specific yellow coolant. You don't. Just use the top of the line Prestone that is for all engine types. Don't add water!! It runs on pure coolant... THE RED OR ORANGE ONE
My understanding is that you should not mix the Red and Green antifreezes. If your vehicle has Red currently. Flush it out completely before adding green. If you just need to top-off the fluid then put only the Red kind in. I believe that the brand is Pentosin http://www.car-stuff.com/exclusives/vw/jetta-antifreeze.html (the dealership or a volkswagen service shop can tell you exactly what brands work).