Soak your rockwool cube in pH adjusted water for 1 hour or so. You can also buy conditioning solution from a hydroponic store that will adjust the pH of the rockwool. Then once you are ready to place the seedling in the rockwool, take the rockwool and give it 3 or 4 hard shakes to remove the excess water. Do not squeeze the rockwool because doing so will create air pockets which you do not want. As for nutrients, wait till your 2nd or 3rd set of leaves form.