From Milano Stazione Centrale (the train station where you arrive) get out from the main entrance on the RIGHT (looking at the station Square) side of the Station building. You will see a bus terminal. Take the bus 87.

You need to buy the bus tickets in advance, since they are NOT SOLD on the buses. You can buy them from any newspaper shop while you are inside Stazione Centrale. I suggest you to buy at least 2 or 3 tickets: one to get here, one eventually to go to the hotel, and the last one from the hotel to the station. It should be 1 euro each. You should stamp your ticket in the little yellow machines in the bus.

You should get off near the Greco Station, right before the Arcimboldi theater (Ask the driver for Teatro Degli Arcimboldi)

Walk back and when you cross two streetcar tracks, make a right turn. You will end up in a large square, traversed by the streetcar tracks (you wack in the track direction) with two large U-shaped red buildings facing each other. Keep walking until you see the Physics department entrance in front of you, in the left building

Ask for me at the reception, or go to the 5th floor with the elevator on the right, my office is number 5041.

You can easily find previews of the place where you get off using google maps, there are nice photos of the place there, look for Teatro degli Arcimboldi, and Università Bicocca Scienze. Take a look at the map.