You normally add it to the mash and sparge water. If you use a software package (I use Promash), then it will give you a table showing the amount of each salt to add to your water to reach a particular style, like Burton water. You obviously need to know your water profile before you start.
The resultant water is never exact to the figures that they provide - in all cases, near enough is good enough.
i remember when i was a little kid living in adelaide, i couldnt see to the bottom of the bath through the brown muck that spews out of the taps down there...hope things have improved.
anyway go easy on the magnesium sulfate. ive made a couple of undrinkably harsh bitter IPAs when i got a bit overeager with the MgSO4.... you probably know this already but you want to use mostly Calcium sulfate (gypsum) to get your water hardness. here in sydney i find about 20g of gypsum and half a teaspoon of MgSO4 per 20L batch to be about the maximum before it gets nasty. at the right level it gives a nice juicy bite to the hop bitterness.
anyway get a hold of a water analysis and do the calculations - if you don't have promash there's good stuff for that in the ray daniels book and in howtobrew.com.
as pedro says just stir it into your sparge and mash water.
hi guys, just to update some of you here on the water in adelaide,they did a test on water from all the capital cities going back about 2 years ago and the water in adelaide came out as the second best in australia,this used to be the story 25 years ago that adelaide water could be a bit rough but its the same old story that once you get a reputation for somthing its hard to change peoples minds about it. the water here is beautiful now with all new filtration plants,at least ours are all modern now unlike some other cities that are still using 60,s technology.oh and just for the record sydney had the poorest water,HMMM WELL I GUESS AT LEAST THEY HAVE SYDNEY HARBOUR THAT LOOKS NICE