I have never rehydrated yeast for any kits. It's simply not necessary. If you do rehydrate in the recommended 104°-109° water, you want to watch the temperature difference between the rehydrated yeast and the must. A temperature difference of 20° or more will shock the yeast and most likely kill most of them. If your yeast cools down to say 90° and you add it to 65° must, you're probably not going to see any fermentation.