Everyone's totally correct here. Be sure to walk over to Cedar Street since it's by far the easiest way to get to Harkness. There are two Harkness buildings, so you'll want to go into the taller one. (It'll have a playground in front of it to the right.) The elevators are straight ahead from the main door. They're right in that you take the elevators up to the second floor, left out of the elevator, walk VERY briefly (like 2 steps), then right. Once you're on the second floor, you can't get lost.
If you see someone who is around Harkness, they will probably know where medical student interviewees are supposed to go. (If you ask for directions on the street, you might want to specify that you're a medical school interviewee since we also have residency interviews going on intermittently.)
Good luck tomorrow! You'll do great.