If you want to come by bus from Trieste center, you should take the bus 36 at the train station (Viale Miramare) or at Piazza Oberdan (see the map of the railway station zone). You should stop just after the two tunnels and then use the map of the campus to join SISSA.
Coming from the Airport, you should take the bus 51. The bus has a stop at Grignano (very close to the stop of the bus 36, see the campus map) and another one at the railway station of Trieste. To get precise information, you can go on the web page: