Hey
Try to put the leg with a temporary splint. There are many websites that help to show how to do this if you do a search.
I found a few that might help...
http://burgebirdservices.homestead.com/er_break.html
http://www.starlingtalk.com/fractures.htm
http://www.petcaretips.net/bird-first-aid.html
You'll need to find the break in the bones but be careful not to cause any further pain or distress. If your not 100% sure of how to do it properly then don't risk hurting the duckling.
The duckling can be fed on corn... wheat or mixed grains.
You should be able to buy these from any good pet shops.
Foods like bread should only be used as a last resort and not given in large amounts as it swells inside their crops ( the organ for storing food before it enters the Gizzard (Stomach)).
Make sure it has plenty of water... although don't let it get too wet as it could get a chill and die as ducklings are not totally waterproof at a young age.
The best thing you can do it try to contact any Vet's or wildlife rescue centers that could help. Even any local bird breeders may be able to help you by giving advice or help bandage and splint the leg.
If you get anymore probs please write back... let us know how it go please.
Mel x