Sorbate won't stop an active fermentation. You will have to first cold crash it to drop the yeast into dormancy and then rack off from the yeast. Then you can add your sorbate to stop the yeast starting up again.
When adding sorbate I usually take a sample of the mead, put it in a small pan and add the sorbate and crushed camden tablet. I lightly heat the mixture and gently stir until all the chemicals have fully disolved, then put it in a fresh carboy and rack on top of it.