Honestly, it depends on your build. I have worn dresses and worn my pump in my bra. I am not a "big" girl on top, so hiding a pump was a little of a challenge. I wore my pump on the side, under my arm. I have a little soft pouch I put the pump in to avoid plastic poking me and it worked pretty well. If you were to really look, you might see it a bit, but since it was under my arm, it was barely visible. A couple people have used the thigh strap for dresses. I have not tried that, but I am sure it would work pretty well.
The other option is to just have a small button hole in your dress and let it show. Heck, it is only a pancreas!