Camden tablets (Potassium Metabisulfite) is used to stall/stun bacteria while your yeast can get a foothold and begin a good fermentation.
Whether a recipe does or does not you should use this prior to the addition of pectic enzyme (@12-24 hrs), then later on your yeast (again @12-24 hrs).
You will use 1 tablet per gallon of must/wine to be made. Disolve after crushing the tablets in a few drops of water or even better your must, then add to your must and stir well.
Place a straining bag or loose lid on top of your primary and wait 12-24 hours before proceeding with the recipe.
Never inhale the fumes from the camden tablets.