We use Bimastat calf scour treatment as recommended by my vet. It is in a pink box and has worked well on the occasions we have needed to use it.
However, it is important to ensure there is no underlying infection as foals get dehydrated very quickly so if it is no better within 24 hours call the vet.
Agree with RSC. Although we mix the forgastrin with kaolin as well. Probiotics as a drench for the first 5 days after birth seems to minimise incidence of scouring anyway. Check the mares bag to ensure that the foal is sucking, as Irishlife says they can dehydrate easily.