I thought there used to be pictures of treating this somewhere (but can't seem to find them now). When I took our first one to the vet, he used tweezers to pop the "plug" (the black spot) out of the swollen area and then he squeezed yellow infected stuff out of the opening (it was more the consistency of curdled milk than puss). He gave me antibiotics to give her twice a day - but he said that the antibiotics weren't necessary if the the infection hadn't gotten really bad. Then he suggested that I keep her in a smaller enclosure with clean straw (or other bedding) for a few days to keep the dirt and mud out of it while it started to heal. He wanted me to squeeze the infected area every day, but I never did get much more out than he did at the vet clinic.
That's my experience, don't know how much it helps. Hopefully there are others that might have suggestions as well, or can point you to more specific treatment options.
The other thing I did was to clear any debris that could cause injury out of the duck run and rake the area that they free range in. I had some left over broken up pieces of a cement patio that we removed and there were also a lot of brambles around their pen and exercise area so I cleared those out as much as possible. I think this helped prevent future problems.
