I fed my khaki campbell ducklings waterfowl chick crumbs upto 6 weeks of age then I have been feeding them a growers pellet. If you go to your local farm store they will be able to advise you.
If the duckling is moving away from the heat lamp, is laying dow alot and panting then the temp is prob too high, if the duckling is huddled under the heat lamp then too low a temp. moving around and sitting in different places usually means the temp is right. the link that Mo gave says, The duckling will need a temperature in the brooder of about 86 degree fahrenheit day one and then by day 7 about 81 degrees.
Once your duckling is dried out and strong enough it won't rely on the heat lamp like other birds such as guinea fowl.
Good luck with your duckling.
Sara @ farmingfriends