I've never heard of that particular kit, but I would recommend against adding sugar. Some beer kits actually instruct you to add sugar, but it's not the way to make the best tasting beer. For people who wish to add something, you could add more malt extract. That does cause some inbalance in the beer with the malts and hops, but it is one way to boost the alcohol.
The difference between an ale and a lager is fermenting temperature. An ale is fermented in the 65-70 degree range, while a true lager is fermented around 50 degrees and then lagered (cold stored) for a couple of months. The difference is not in the ingredients (and a good lager should never have sugar as one of the ingredients) but in the way it's fermented. Some kits call themselves "lager" kits, though, which makes it confusing. Those kits are usually still sold with ale yeast, though.
Some good reading on extract brewing is here: http://howtobrew.com/intro.html
When you go to bottle the beer, some more fermentables (usually in the form of sugar) are added. That's just to carbonate the beer, not to actually provide fermentables in the brewing.