You can get to the Observatory from Harvard Square up Garden St (red path, with cyan one-way detour if you are driving) or from Porter Square (green path), down Mass Ave and then right on Linnean. The Obs entrance is at the intersection of Garden and Linnean. You'll see "Perkin Lab for Astrophysics" at the entrance, don't ask why. See the detailed map below.
Drive up the hill past the initial parking lot and round to the back (main) lot (outlined in red). Park and walk up past building D (marked "Phillips Auditorium") and building A along the path to the building B entrance where we'll let you in. The telescope is on the roof of building B.