Coffin Rock
This mill lies nearby "Coffin Rock", the infamous rock in The Blair Witch Project where the myth said a rescue team was found mutilated, but later disappeared completely.  According to directions provided by BlairWitch.net (see below), you park at the old mill and walk across the road to the barrier.  There will be a trail visible and that is the trail that will lead you directly to the location used in the film.  Be sure to walk the trail on the side of the road away from the mill with the creek on your left.

Past Vandalism
Please be aware that The Blair Witch Project brought a lot of notoriety to this and several other locations.  With that notoriety came a lot of tourists and vandalism to several of the key locations.  If you want to visit this location, please be respectful of the location and the people who live nearby.  It really isn't too much to ask and any acts of vandalism will only taint the location for future visitors.