I just added it to my mead batches as needed, without microwaving, boiling, etc... Batches (all 4 of them) are coming along just fine... I will say to sanitize what's going to touch the nutrient (as much as you can)... Putting the nutrient into liquid usually makes some stay behind in whatever you put it into (hence not doing it after a couple of times)...
If you added some when you were getting the cider together, and it's already fermenting (you just want to give the yeast more nutrients) then you should be fine (your place is clean, yes?)...
I've never seen any mead makers post up sanitizing DAP when they add that to their mead (the Wyeast nutrient is almost like DAP, only you'll need to use more of the Wyeast product)...
I will say to measure the nutrient with a scale (if at all possible)... ~1-2g/gallon is usually what you're looking to add (at each addition)...
Personally, I'll keep using the Wyeast nutrient in my beer, but I'll be using DAP for any future mead batches. That includes the hard lemonade I'm planning on making now...