Take Route 28 north (the freeway that goes up the Allegheny, past the Waterworks) until you get to the Orange Belt at the Creighton/Russelton exit. Follow the Orange Belt north (towards Russelton) until just before Russelton, and turn right on Mahaffey Road. (There's a low bridge over the Orange Belt immediately after where you should turn off of it.)
Follow the road for quite a ways until you get to the top of a big hill, with a grassy field/parking lot on your left. Park there; the observatory is the rectangular building at the other end of the field. Turn off your headlights as you leave the road, so as not to blind the astronomers (leave your parking lights on and go really slow).
The phone number of the Wagman observatory in Deer Lakes Park is 724-224-2510.