I think it's a bit too early to hear heartbeats and I suspect it may be another week. I heard mine with a stethoscope around 43 days or so. You should place the doppler along her teats on each side. These are where her horns are located.
A good site ...
Canine Reproduction Video and Book on Breeding and Whelping dogs by Debbie Jensen for dog breeders and Whelping Supplies needed to deliver puppies.
I really urge you to buy the book Puppy Intensive Care by Myra Savant Harris. It is full with lots of valuable advice on the supplies you need, the whelping, and afterwards.
Be sure to speak with your vet. Will they be available 24 hours a day or will you need to take her to an ER hospital if necessary.