First step: Go from train station to the closest metro station to the university, namely Cité Scientifique. There are two main train stations in Lille. If you come from train Station Lille Flandres, the metro line 1, direction "4 Cantons" drives you directly to the stop "Cité Scientifique". If you come from train Station Lille Europe you first need to take the metro line 2, direction Saint Philibert, for one stop and change at Lille Flandres and continue with metro line 1 as indicated above.
Second step: walk from metro station Cité Scientifique to the conference building (which is the math building). Continue walking following the (overhead) metro (again direction 4 Cantons). Then you can see, on the right, a high building with a very high radio aerial (antenne radio) on the top. This is building M1, and the talks will take place in building M2, just opposite building M1. The talks will be in the "salle de réunion in the 1st floor (recalling that according to European standards, the ground floor is zeroth floor).
Some useful maps can be found here