No tools are allowed, no breaking of rocks are allowed, no climbing up the sides of the spillway, fossils cannot be sold, and fossils larger than the palm of your hand have to be left in place and the office notified for collection. Permits are free and can be obtained at the main visitor office. Parking is plentiful at the spillway.
I recommend taking some good hiking boots you don't mind getting muddy.
Best time to find the best fossils is early spring, as the freeze/thaw over winter helps break free new fossils. Throughout the year the ground gets picked pretty clean. Although you cannot fully climb the sides of the spillway, you can still clamber up pretty far before it becomes "climbing." Rules are in place as much for personal safety as they are to prevent damage to the spillway. Large slabs of rock are present, and it can be easy to trip and fall as you rockhound. Some of these slabs are chock full of fossils, and it's easy to image them littering the ocean floor millions of years ago. Being a spillway, it's mostly flat and can be quite muddy, definitely recommend a change of shoes if you don't want mud in your car. I've also only ever found a neat rock as big as my palm, the fossils are much smaller.