If there is already a case, then contact the lawyer or law group in charge of the class action suit. All the filed papers should have a lawyer contact.
If you are trying to start your own then here:
It's extremely hard to prove.
You'd need have the dog food tested for contamination. You need to gather other DOCUMENTED cases. In some areas, you can run an ad in the paper looking for others who have lost dogs in this manner.
You'll probably need vet records, blood, perhaps even a dog autopsy.
You can't just guess the dog food did it, you need to get proof.
And then you need to get a lawyer.