From Wikipedia, "The pinewood derby is a racing event for Cub Scouts in the Boy Scouts of America. Cub Scouts, with the help of parents, build their own cars from wood, usually from kits containing a block of pine, plastic wheels and metal axles ..."
The Stumptown 40 is a lot like a Pinewood Derby, but instead of boys creating cars from blocks of wood, it's Portland's creative types. And instead of a church gymnasium, the Stumptown 40 will be held at Spirit of 77 (where a full bar is available). Car kits are $25 (price includes event admission). Register here.
Hey, it's for charity, man. We generally select a different charity every year. The 2015 race's net proceeds will be donated to the Multnomah County Sheriff's Office of Search and Rescue (MCSOSAR), where co-founder Blu volunteers on the weekends. MCSOSAR is a 100% volunteer-staffed organization on call 24/7/365. They're the people that come find you when you get lost hiking.
Not so good at questions, either! If you're not the whittling and/or sanding type, feel free to join the fun as a spectator.
Unlike derbies of the Kentucky sort, the Stumptown variety requires no fancy hats or party gear, though such attire is not discouraged. Consider something more utilitarian - perhaps a belt on which you can hang a pocketknife (Whittlin' Chit required). A jaunty neckkerchief will appeal to both the aesthetes and the mathletes in the crowd - it works equally well as neckwear or emergency tourniquet.
You're right to be excited. Participating in the Stumptown 40 is about the best thing a person can do.
Print the receipt you received when you registered, and bring it with you to Hand-Eye Supply (427 NW Broadway, Portland, OR, 97209) between 12 PM and 6 PM, 7 Days a week. Show one of the friendly staff members your receipt to get you your kit, and you'll be ready to start sawing, sanding and smoothing!