one is a little pricey but well worth it in my opinion.
Adding sugar to a recipe will increase the alcohol content and dry out the beer. The advice previously given to you by inhousebrew is solid. Don't add more than 1 pound. The sugar is very fermentable and if you add too much your brew will get "hot" or "boozey" and take a while to age out. The sugar is also very easy for the yeast to eat. If you feed them too much sugar they will choose to consume that and have a hard time consuming all of the harder to digest sugars from your extract. You can add it near the end of the boil or wait until a couple of days into fermentation. If you choose to wait, boil it in just enough water to dissolve the sugar, cool it to the same temperature as your batch, and pitch it in.