The observatory is located on the roof of Herzberg Laboratories, at the top of stairwell C. This stairwell can be accesed from any floor of the building, or from the Shipping & Receiving entrance on University Drive. Elevators located on either side of stairwell A (the main stairwell, with the pendulum) can be used to reach the top floor, but stairwell C must be used to access the rooftop.
The main doors of Herzberg are sometimes locked at night or on the weekend. They usually can still be opened by using the ♿ button; if this does not work, the building can be accessed through the tunnels, or through other ground-floor entrances.
These directions are illustrated in the map below. Directions to Carleton University can be found here, and the location of Herzberg Laboratories here; both can also be found on Google Maps. Additional parking options can be found here.