Forget the pumpkin.
I have used it before and it does work, however it works just by adding more fiber into the poop, doesn't really help the situation.
What actually works much better, faster, and helps with the cause of the runs is slippery elm. You can pick it up at most health food stores and it works like an anti-inflammatory on the dog's gut. It solves the runs and soothes their tummy, it works much much faster then pumpkin, usually within one dose and actually begins to help the gut heel.