Get off at Firenze S.Maria Novella railway station. Then the easiest way is by taxi (around 15 minutes in normal traffic conditions).
If you want to go by bus you can take the bus number 11 (the bus stop is just outside the railway station,tag number 17 or 18 on the MAP ) to the first stop in Via del Gelsomino (tag 26) from where you have to walk in Viale di Poggio Imperiale to the main gate in Largo Enrico Fermi,2 (around 10 minutes or 800m). Hint: zoom in the map to view street names
From the main gate to the Institute you have to walk for a little until the top of the steps. Once inside you will find the indications for the Institute