I would add 4 days before you bottle/keg.
If you have one of those fine mesh syphon end bags (the sort that come with the Festival kits) then you can just chuck the hop pellets into the brew loose. If not I would make up a hop bag out of some sterilised muslin, string and some weights (marbles are good) put the pellets in the muslin with the marbles and tie it shut leaving a bit of space for the hope pellets to expand when they absorb the wort - then pop that gently into your wort, should sink because of the included weights. Make sure you sterlise the muslin and the string and the marbles.
The aroma from dry hopping doesn't hang around that long in the bottle/keg before subsiding with a bit of time, so adding 4 days before bottling/keggin would be my advice.
Good luck with it.