The dog park has been open for close to two years now, and provides a fairly large wide open space for dogs to run and stretch their legs.
The pond/mud pit that used to be open is now fenced off with a gate so you can let your dog in to romp around the mud/water if you wish. A sign asks people to keep the gate closed so people who don't want to go home with a wet muddy dog can keep their dog out of the water.
If you do let your dog play in the mud/water, there's a hose available to spray him off with.
Picnic tables provide plenty of places for people to sit down and watch their dogs, or you can hang out on the concrete walkway along the fence on one side of the dog park. The trees provide a bit of shade there for sunny days.
Admission to Wannamaker County Park is $1/person and once you're in you can make use of the dog park, go wandering around the trails afterwards or bring the whole family and have a picnic. To get to the dog park, just follow the road once you're past the entrance gate (the road goes around the water park). You'll see signs pointing to the dog park.
You can see the rest of the pictures here.
Here's a Google Map to the dog park
View Wannamaker County Park dog park in a larger map