adding lactose
 

boil it in water or dump it in dry ( secondary or keg)

When I brewed my cream stout, I added it to the boil with 10 minutes remaining. I believe that this is the preferred (accepted) method.

I usually add my right around flameout. I would boil it before adding.

I boiled it for 10 minutes, just to be sure it is sanitized and dissolved.