you can add it whenever...it's basically just minerals, vitamins, inorganic phosphates, etc. Stuff the yeast need to build healthy cell walls when they are in growth phase.
I'm assuming you're making mead or something similar...as a general rule, nutrients are not needed for beer making, although you could make a case for it if you're brewing something really high gravity and the yeast need all the fighting chance they can get...
If it's mead, you should be adding some at the beginning, and then in additional doses during the first phases of fermentation (check out "staggered nutrient additions")
Packaged: Red X Weimea SMaSH
Recent Meads: Mead Day '11 Ginger Metheglin, Mead Day '12 Traditional (orange blossom) Mead, Simple Cyser '12
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider, Simple Cyser
Planned: Hop Metheglin #3 (NZ hops), Trad. Gesho T'ej