The seeds are indeed edible and apparently you can eat the young leaves, but they need cooking to destroy the calcium oxalate in the leaves. Cooking or drying render it perfectly safe though.
Here is a recipe for Himalayan Balsam seed curry.
Here is a recipe for using the flowers in salad and to make a sweet jelly.
Oh, and if you have any of that other rampaging invader, Japanese Knotweed, you can eat that too. My friend loves it and says it's like rhubarb, only nicer. This website agrees with her and suggests some ways of using it.