Hidden Valley Observatory (HVO) is located at 4715 Hidden Valley Road, Rapid City, South Dakota. The driveway to the observatory is marked by a post with a green address number sign of 4715. It is ~100 yards on the gravel driveway to the observatory. The nights of public workshops a larger sign will be present at the gravel entrance.
For a view of the property with parking and other amenities noted, see this page