I reckon that with extract it's probably best to add it just before bottling, especially if you're looking for lots of flavour and aroma from it. That way there's no danger of aromatics being blown off with CO2. Also, in my experience, the flavour of some extracts fades with time, so adding a bit later can't hurt.
Add it to the bottom of your bottling bucket with priming sugar, rack your beer on top of the lot, then bottle as normal.
Primary: Beast of Burden hoppy porter/black IPA
Bottle Conditioning: Nothing
Drinking: Buff Face Imperial Stout, Cria (2.X% ABV hop bomb), Llama Doodle NZ hopped IPA