For pasteurized juice, I don't add Campden until the first racking. For fresh/frozen fruit, I add Campden when I mix up the must, let it sit for 24 hours and then add yeast. Thereafter, I try to add campden (1 tablet per gallon, certainly no more!) at EVERY OTHER racking. I don't have an so2 meter, but your goal is to keep the sulfite at 50 ppm. Also, campden should be added at bottling. Most wines are racked at least several times
I like the directions treemuncher gave about crushing them- they are tough little buggers!
Campden does not inhibit or kill wine yeasts, so it's no problem using them. Still, most recipes tell you to wait 24 hours before pitching your yeast when you use Campden.