Thanks for the heads up, Alan. Only 3 days notice is not much!
Russ, you'll need all info about your car, like make, model, color, engine type/size, insurance, plate number, etc. They'll also want info about driver and passengers, like how many passengers, who will be drivers etc. And have your credit card handy for the fee. Go slow enough that you don't make a mistake and have to go back to fix it. Lots of stuff can be fudged, like make/model etc, because you are not held to those. In other words, you can show up with a totally different car and still get in. You just want to get the info filled out, pay the fee and register. But like Alan said, nobody has ever been turned away as of yet, so I wouldn't worry if you miss the deadline and have to get on the waiting list.
Edit: Another bit of advice, book your hotels as soon as possible. They fill up quickly. I booked mine 2 weeks ago, and the hotels near the tracks were almost filled up then. Depending on the track, you may have to drive an extra 10-15 miles to get to it if all the hotels near are booked up. And don't be surprised at hotel prices. They know a big event is happening, so rates automatically go up!