From what I remember, it's added while you're heating the malt extract. Just stir it in, and you'll be set. It provides fermentable sugars for the yeast without contributing much flavor. It adds about 1% ABV. For a better beer, I'd recommend ditching the booster, and picking up a 1.5lb package of DME. It's going to provide fermentable sugars, but it will also improve the flavor and mouthfeel of the beer, unlike the booster.
Once fermentation is finished, you'll still have to add priming sugar to the bottles to carbonate them.