The subway line U6 goes to the Forschungszentrum Garching. From the central station (Hauptbahnhof) you can use the U1 toward Mangfallplatz and change trains at Sendlinger Tor. Another possibility is to take the U5 toward Neuperlach Süd and change trains at Odeonsplatz. The trip takes about 30 minutes. The ticket costs 5.20 euros (single ticket 2 zones). The City-Ticket by the Deutsche Bahn is only valid as far as Fröttmaning. You can find further information on the MVV web page.
Leave the motorway A9 at the exit number 70 Garching-Nord. After leaving the motorway, turn right if you are coming from Munich or left if you are coming from Nuremberg. At the next intersection, turn left onto the B11. Turn right into Lichtenbergstraße at the following intersection. In front of the gate of the Forschungs-Neutronenquelle, turn left into Boltzmannstraße and at the end of the street turn right into Am Coulombwall. There you will find the parking lot of the Fakultät für Physik. You can find a map of the route here.
The closest airport is Munich Airport. From there you can take the S-Bahn number 1 toward Laim. At the stop Neufahrn transfer to bus number 690 toward Garching, Forschungszentrum. The trip takes about 40 minutes. The ticket costs 5.80 euros (single 1-day ticket outer area). Please note that this bus does not run often and mainly on weekdays. You can find other connections on the MVV web page. A taxi ride costs about 35 euros.