Miss Lydia,
My apologies...I certainly meant no offense. But I'm trying to break this down.
Since they usually ship ducklings before they are even eating, and the poster says his duckling arrived with the leg problems (before it was even eating), I thought it veers to more of a genetic problem. The duck's egg might have missed being turned the last couple days before it pipped. Or it could very well be a niacin deficiency, which would commonly cause bowed legs, and rarely a slipped tendon (which could be the cause of the deformed looking foot). By all means, I agree, giving the duck niacin is a good idea.
What concerns me is that whatever the cause of the duck's leg/foot problems, the duck is 2 months old now. It's no longer a duckling, it's a juvenile. If the duck has a slipped tendon or a deformed foot, there isn't much time left to try to reposition the tendon and brace the deformed leg, before everything sets and the duck is left lame. I thought seeing some pics would help.