Brewing salts are added to 1) improve your mash chemistry (e.g. lower mash ph, increase calcium for efficient conversion, etc.) 2) to change water chemistry so final product will have a specific taste (e.g. increase sulfate to enhance bitterness in a pale ale).
If your mash PH and chemistry are in line for your brew, then add the salts to your boil to achieve a specific water chemistry. If your mash PH and chemistry are off, add the salts to the mash (you need to use a calculator for this or do more research on how much for your specific water chemistry). Per some smart brewer who wrote a beer book, most calcium won't make it to the boil kettle after sparging, so many brewers will add some calcium chloride to both the mash and the kettle.
Anyways, that will get you started.
