thanks for responding,
 
We just fed Duck some yogurt mixed with egg yolk. We got about three syringes down. How often and how much should  I feed her every day?
 
When mine are really sick, I tube feed them a minimum of twice a day, how much depends on their size, weight and if they're drinking. There is a very good thread about tube feeding here: 
https://www.backyardchickens.com/t/722041/how-to-t-feed-a-sick-chicken-and-give-subcutaneous-fluid#
http://forum.backyardpoultry.com/viewtopic.php?t=7933
 
I had checked her for mites and could not find a thing. However another member on BYC suggested that she might still have mites and we just cant see them. So I bought Diatomaceous Earth and dusted her all over. (I figured this would be easier on her than using poison) - she is still biting herself and scratching and I am not sure if its from dry skin caused by DE or if the DE maybe did not work?
 
DE doesn't work. I use poultry dust on my chickens and it kills them within minutes.
 
Where did you get the Ivermectin ?
 
You can get Ivermectin at Tractor Supply.
 
This all started about a week ago - I noticed that she was grooming a lot. When I picked her up to check her, she was really skinny (she has always been on the skinny side and is also behind on growth compared to her 2 sisters - she is 6 mo old and has not been laying yet.
 
You should weigh her daily to see how much weight she is losing/gaining.
 
Any way I noticed that she was kind of stumbling, as if she was loosing her balance - so I separated her and took her in the house. She has been there for a week now. I got her to eat some and she seemed a little stronger until yesterday, when she didn't want to eat.
 
The stumbling could be from her not eating and getting weaker.
 
( She took a few bread crumbs and she takes some greens, but this is about it. I don't think that it is some virus ect - because I am sure she would be gone by now.  Besides the weakness and the stumbling, she is very alert, talking and tries to walk out of the cage every chance she gets on wobbly legs. But once she is out, all she does is groom.
 
Is she drinking? Can you post any pictures of her poop?
 
It is very frustrating to try and coax her to eat something, anything - but she did eat some of the mixture in the syringe, so maybe this is the way to go.
 
The best way to force feed is with a feeding tube.