Usually you add the can plus 1kg of sugar when you make up the brew (prior to boiling or fermenting etc). You then use a small amount of dextrose (corn sugar) during bottling.
If you use 1kg of sugar or corn sugar, you'll get a thin, not very malty sort of beer. If you replace the 1 kg of sugar with 1kg of dry malt you'll get a more malty beer with heavier body and better head.
Sometimes, the yeast can't handle a full malt beer, so try experimenting with 500 grams sugar and 500 grams malt and see how you go.
Kits in Australia, say not to boil the ingredients as you can stuff up the hop flavours and bitterness etc., but I'm not sure what your kits in the US are like.