I was waiting for one of the old sages to reply, but since no one else has... if your recipe doesn't specify, I'd go with the main boil. But, you can pretty much add it at any point up until you bottle. Just make sure & boil it in a little water to dissolve it. (& sanitize) if added during the later stages. Good luck. Oh yeah, just to make sure, lactose is "NOT" fermentable by beer yeasts. Ciao.