At the motorway intersection called "Mainz Süd" (A60/A63), proceed towards the center of town ("Zentrum") and follow the signs that say "Universitätskliniken".
For more specific directions use the routing feature of Google Maps.
Starting at the Mainz Central Station take one of the following tram lines departing from the platform in front of the station building's main entrance:
Get off the tram at Gautor station (it is the third stop).
From here, walk into Obere Zahlbacher Straße, wich is righthand (looking in the driving direction of the tram). After you crossed the pedestrian lights, the building on your right is the IMBEI, followed by a multistory car park. Just behind that you will find the Department for Research and Teaching.
The busses directet towards the University Medical Center depart from the platform called "Hauptbahnhof E" located at the near end of Bahnhofstraße. Conveniently, all lines departing form this stop will take you to the University Medical Center. In particular these lines are:
Get off at the stop called "Universitätskliniken". To reach the Department for Research and Teaching, cross the street and walk down the Langenbeckstraße (in the driving direction of the bus you just left) for approximatly 100 meters. The department's building is on the left.