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: Dark Munich Citra SMaSH, Mayan Stout, Caramel Quad, Basic Spiced Cider, Spur of the Moment Graff
Recent Meads: Mead Day '11 Ginger Metheglin, Mead Day '12 Traditional (orange blossom) Mead, Fresh Simple Cyser '12, Cherry Melomel, Belgeglin, Bochet
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet
Planned: Hop Metheglin #3 (NZ hops), Trad. Gesho T'ej