The Harvard College Observatory (and the Department of Astronomy) is located at 60 Garden Street. Garden Street leaves Harvard Square by Cambridge Commons, Here is a map showing the location of the Harvard-Smithsonian Center for Astrophysics relative to Harvard Square:

The main entrance to the CfA is up the driveway opposite Linnean Street (by the Radcliffe Quadrangle and just past the Radcliffe Quad Athletic Center). Go up the driveway and the large 3 story building is the Perkin Laboratory for Astrophysics. The Pratt Conference Room is on the ground floor or Perkin. Enter the building, the Library should be on your left. There is a black doorway just in front of you (right opposite the entrance) which leads to a stairwell. Go down one flight to the G level, turn right as you leave the stairwell and Pratt is at the end of a short corridor.