Can you use store bought Distilled water?
Yes. Reverse Osmosis water works the same as Distilled water.
When should I add the treatment to my brew?
Before the mash. You want the water in your mash to be treated foremost. It's suggested to treat all of the water for your brew but will still be making an improvement on your brewing water if only added before the mash.
Can I use Waters Brewer in extract batches?
You can, but the main importance in brewing water is for the conversion, during the mash. It's still better to have "IPA Water" for an IPA then it is to NOT have "IPA water" at all.
Do I need to treat all of my brewing water or just my mash water?
It's suggested to treat all of your brewing water to get rid of the "well maybe if I added water treatments to the sparge as well..."
Will Waters Brewer make my beer better?
If you brew the same beer over and over and it is consistently NOT bad, then water treatment will definitely help get that beer from good to great. Bad beer is bad beer, but treating your water will give you an extra 10% that you otherwise wouldn't have.