When I added half the F-pack for my Green Apply Riesling, I added to the must and waited for 24 hours before pitching the yeast. I did this as a precaution in case there were any preservatives in the F-pack that might inhibit my yeasties from doing their dance, and also so I could double check the SG before I pitched.
If you haven't made an Island Mist kit before, one precaution I would give is that the taste from the F-pack is super strong prior to bottling or bulk aging. With my Riesling, I was worried because, even though I only used half the F-pack at the end, it still tasted like green Jolly Rancher. After 3 months since pitching the yeast, the artificial apple flavor died down, and after 6 months, it was good enough to share with friends. It's 10 months old now, and it went over so well at Christmas, that I preordered the Pink Lemonade for drinking this Summer and Fall. If my family likes it this much, they can drink my $50 kit and my Eclipse kits can keep aging