Take the clothes from the unconscious butcher in front of you. Exit the truck and close the truck doors behind you by pressing a red shining button the side of the truck; if you don't, the butcher will inevitably wake up and run inside, blowing your cover. Drop any firearms you are carrying. You will have a meat hook, which you can keep (but will not need, plenty more weapons are available). Go around the building and you will see main doors where you will see two guards and some guests getting in party. Approach a guard and wait for him to frisk you ("feel your flesh"). Enter the double doors and you have gained access inside the slaughterhouse.