2# of raspberries for a 5gal batch is a good number. Add them to your carboy, then rack your beer on top of them. No need to pasturize or puree... within a week they will turn white and sink to the bottom.
I would try to get fresh. if you want to get frozen, make sure you check the additives.... I bought fresh, froze them overnight, then racked, and let it sit a week before kegging.
This is my Requiem Raspberry after 36hours on the berries.