I've been to Salzburg a few times, but I don't recall any MAC store (or department counter) in the city. It's a rather small city, so this does not come as a surprise. My guess is that the closest one is located in Munich (about 1 - 1,5 hours drive). Are you traveling from the US? If so, MAC products are much more reasonable priced in the US than in Europe. Enjoy Salzburg, it's a lovely and cute city, but it's not very fashion-forward.
PS: Maybe a local might have more insides if there is a hidden store that sells MAC.