The egg sack gives nutrition until about 3 days old, that's why they can ship day-olds via the post office. They should start eating soon. Are they lethargic at all?

Try tapping your finger on the food, imitating the mother duck. Perhaps they just need to be taught that it's okay to eat what you've put out. Try teaching them to eat. What is Flock Starter? Muscovies need a high protein diet, but too much can cause angel wing.

Worms. Put wiggly, live earthworms or red crawlers in their dish often-it should get them hopping & chasing each other for them. If they don't I'd be very surprised & agree something is wrong.

Brendasue