Originally Posted by z987k
I'm kind of looking into this a bit myself right now, but campden tables = potassium sorbate. Potassium metabisulfite will kill the yeast, k-sorb stops renewed growth.
Sorry, that's wrong. Campden = potassium metabisulfite and does NOT kill yeast!
Read your source more closely- k meta inhibits yeast growth, never kills them. Potassium sorbate also does NOT kill yeast; k sorbate inhibits yeast reproduction. They are most certainly different chemicals, but work best together.
The best way to back sweeten is to wait until fermentation is completely stopped. (Because neither of those compounds actually kill yeast, it's really tough to stop a active fermentation, which may be why you have airlock activity- you may have just temporarily stunned them). After fermentation is stopped, dissolve 1/2 tsp per gallon of k-sorbate and 1 campden tablet per gallon into some boiling water (like 1/2 cup) and stir until completely dissolved. Put this into your carboy, and rack your mead into it. Wait about 3-4 days to ensure fermentation has not restarted, and then sweeten as desired. I would wait another week after sweetening, just to be 100% sure it doesn't start up again. Then bottle.