Getting the water profile you want starts with defining the overall mineral balance you desire. You can set this in the water profile tool or by directly defining a water profile you want to meet.
For the latter, you can open up your water profiles ['profiles' > 'water'] and then create a new profile to meet the quantities of each of the minerals and ions you want in your finished profile. For instance if you select a new profile ['add water'], you can set your Chloride to Sulfate ratio based upon targeting the amount of each you want to have in your water. For a 1:2 ratio you can target Chloride at say 75 ppm and Sulfate at 150 ppm. You can fill in the rest of the minerals as you want. I would recommend a target of at least 50 ppm for Calcium and 10 ppm for Magnesium. This option works best if you pretty much know a profile you want to meet and can enter it in.
The water profile tool allows you to start with your base water (tap, RO, distilled) and build it by selecting a pre-existing profile to match and then clicking on 'match target profile'. You can save the salt additions to the profile if you don't want to have them automatically added in your recipe. If you are using this route, you can easily just do this within the recipe on the 'water' tab and get the same result. You can begin by using the standard color/flavor profiles in the water profiles listing. Here you can choose based upon the basic color (pale, amber, brown, black) combined with the flavor balance (malty, balanced, bitter) which will give you a place to start.
Again, if you know the basic parameters you want to hit for Chloride and Sulfate, you can also use the water profile tool to build up to those target quantities and then save the profile to use within the recipe.
All of the above assumes that you have some understanding of water chemistry to know how to build your profile. If you are looking for some primers on water, you can search the BeerSmith blogs and podcasts for several of the articles Brad has put together or look up 'Water Knowledge' by Martin Brungard for a good primer on how to put together water using the various commonly used available minerals.