Agree with all the posts above. First step is to find where your horse is lame and second step is rest and treatment. If it were me, I'd have the vet out with the farrier if I didn't have an idea of what was causing the lameness, so that you can have a better idea on how to approach it.
Personally, I don't condone riding a lame horse at all. Horses are prey animals and most hide lameness VERY well. Furthermore, horses can develop secondary and even tertiary lameness from compensating. You could do far more damage (even permanently so) by riding since you have no idea the root of the issue. It could be something as simple as an abscess, hoof bruise or muscle strain, but it could also be ligament or tendon damage which are not easy to heal.You won't know until you have a thorough checkup. So, it is better to error on the side of caution.
I would call the riding camp and see if they may have a horse you could borrow (or if anyone you know does) and if not, then cancel and look for another event farther away. Of course it is disappointing to miss out on the fun, but it is MUCH more disappointing to find out that you've gone and made the lameness much worse by attending camp, not to mention the potential $$$ it can cost in future vet bills.