From the train station (northern exit "Gare nord"), to reach the campus, you have several possibilities :
The Tournebrige bus stop is just in front of the campus (main entrance to the University of Rennes 1). In order to get to the campus you need to go down the bus at this stop and cross the street.
From the entrance of the campus, you can reach the INRIA Bretagne Atlantique building in about 5 minutes.
The map on bottom shows the trail from the main entrance to the INRIA Bretagne Atlantique building reception.