The "Cyclotron" or "Bâtiment Marc de Hemptinne" building is located in F8-F9 on the map below, building #1.
By car ...
From the E411 motorway, take exit 8a, then the third exit of the roundabout (left direction), then first on the right : "Boulevard Baudouin Ier". Pass in front of the gaz station (Q8), and use the P3 carpark or "Parking Baudouin Ier" (free) on the left at the roundabout (F8) (do not use the P1 carpark which requires a UCL special label).
By train, walking directions ...
The "Louvain-la-Neuve Université" train station is located at D6-D7 (label SNCB - building #18) on the map.
The building of IRMP is 10 minutes away from the station.
When you get out of the station, you arrive in the street "Rue des Wallons" : move to the left (when the station is in your back), which is also the direction where the street is going up.
Climb up this street which zigzags a bit for 5 minutes.
Go across a first place named "Place des Wallons", then after some more walk climb a few steps in a stair. After a last zigzag to the right through "Place Galilée" you arrive at the "Place des Sciences", a big square with a wooden floor in the middle and a big concrete building with only few windows which is the "Bibliothèque des Sciences" (Science Library, building #7 on the map).
On your right there sits an octogonal building named "Salmigondis" or "Pythagore" (about #28a on the map), just in front of the Bibliothèque des Sciences. After this octogonal building turn to the right. You will arrive on a parking lot. Then you take slightly to the left and (at least during the winter) you should see to the right a grey high chimney. Go towards this chimney to find a path ("Chemin du Cyclotron", the chimney is the little blue square on the map). Just besides this chimney is a small pedestrian tunnel going under the motorway("Boulevard Baudouin Premier"). go across the tunnel. The IRMP building is the first building on your right.
As a matter of fact, GPS' or computer-maps do not precisely locate "Chemin du Cyclotron, 2". Look, in the Maps section below, at the exact location of the institute.