When I bought my 97 SE and was swapping wheels a few months ago, I ended up killing a stud. It was the fault of whomever cranked the wheels on last time with five million lbs of torque. Anyway, to change it, it's easy. First get a replacement stud, and a open bolt (the ones on your car have "hats"; you know, they're curved, so you want one that is see through). I've had luck twice at advance auto parts getting these for a 97 and an 00, but surely a nissan dealer will have them. Parts are about $2.50.
Labor is easy and goes as follows:
1) Lift the car with your tire changing jack (if you don't have a better one...when lifted with one of these, remeber not to trust them enough to put your head under parts of the car that could fall) and take the wheel off. It's doubtful that the tireiron included with your kit will actually take the studs off. The dick who stripped it the first time almost certainly used an air tool and put it on hard enough that you'll need a breaker bar to get it off (but as a man you should have one anyway).
2) Using a hammer, hammer that busted stud until it flies out.
3) With the new stud, put it in place, and using that open nut that you bought, crank the stud back into place. All done!
Tips: You will want to get a washer to go against your wheel hub, so that when you crank that washer on, the nut is rubbing against a washer instead of the wheel (just get a washer from home depot for $.20 that just fits over the new stud). You may also want to grease up the stud and nut, so that they go on easier. Don't go too fast, because the faster you go the hotter the metal will become and it could damage it a bit. Not that I did, but it was recommended to me to throw out the nut that you put the new stud on with, because it will be slightly damaged from all the force required to put the stud on. What I've done the two times is that my wheel was spinning when I tried to crank the stud on, so I had my hammer pointing up from the garage floor perpendicular, and an old stud resting on it (so it was bracing the wheel from turning).
Really though this is easy and if you want to do it, you can't go wrong. Please for God's sake don't pay nissan to do it.