The university is reachable mainly by tramway (line 1), see Venue & Stay.
For instance from the tram stop "Université Montpellier Triolet" you will need to walk 250 m to place Eugène Bataillon, see Map1
Once you enter in the University you have to go the Building 20 (250m more to do), the amphitheater is on the left, the entrance is at the middle of the building, see Building 20
The idea is to cross toward the administrative building (build 7) you can either cross it or turn around it. Then you should see the Library (build 8, BU), move straight, you should see building 13 (the LUPM lab is at the fourth floor, you can see domes on the roof), finally Building 20 is just behind the 13.
You can send a mail at almarcowith @ gmail.com if you are lost.