The Hallam Observatory is located at 3989 South Middle Road (Lat. N42d 12.411m, Long. W82d 34.614m)
The map above shows the location of the Hallam Observatory. Use the controls on the left side of the map to move and zoom in and out. The most direct route from Windsor is to take 401 East to Highway 77 South to South Middle Road and turn right (West). While on South Middle Road you will go about 1 kilometer and just after the point where Concession 9 joins it you will find the observatory site on the left side of the road (South). If you hit the Rochester Townline Road (i.e. you come to a stop sign and have to turn left or right) you have gone too far.
IMPORTANT NOTE: The Hallam Observatory grounds are private property and no trespassing is allowed. Other than the private group observing sessions the site is closed to the public.