How to get to Ruhr-Universität Bochum campus by train:
You will arrive at "Bochum Hauptbahnhof" (see http://www.bahn.de/ for connections). Go to the basement of the railway station and take the subway U35, direction "Hustadt", and get out after about 10 minutes at the stop "Ruhr-Universität". You need a Zone A Ticket (ca. 2 Euro). For further directions see "How to find Theoretical Chemistry on campus".
How to get to Ruhr-Universität Bochum campus by car:
Bochum is reachable via the A43/A1 (Hamburg-Köln) in the north-south direction and the A44 (Netherlands-Kassel) and B1/A40 (Ruhrschnellweg) in the east-west direction. The exit Ruhr-Universität is at the highway intersection A43 with A44 (A43, Exit No. 19). Here is a sketch of the highways for viewing and for printing. The Ruhr-Universität Bochum lies to the south-east from Bochum city center (Universitätsstraße in direction "Autobahn").
How to get to Ruhr-Universität Bochum campus by plane:
The closest international airport is "Düsseldorf International" (DUS). You will most probably arrive at the main terminal building A-B-C. Follow the red "DB" signs and take the "SkyTrain" to its final stop which is "Düsseldorf Flughafen/Airport Rail Station" (ca. 5 minutes journey). Important: there are two train stations at the airport, do not go to "Düsseldorf Flughafen Terminal/Terminal Rail Station" which is in the basement of the main terminal building! At "Düsseldorf Flughafen/Airport Rail Station" you should take one of the Regionalexpress trains to Bochum (RE, 30 min journey) or the local train (S1 direction 'Dortmund', ca. 60 min journey). You need to purchase a Zone D ticket (select "Bochum Hauptbahnhof" at the vending machine; for IC oder ICE long-distance trains you need a different ticket). From "Bochum Hauptbahnhof" take the subway (U-Bahn) line U35 in direction "Hustadt" and get out at the stop "Ruhr-Universität" (see also "How to get to Ruhr-Universität Bochum campus by train" and "How to find Theoretical Chemistry on campus"). Note that the local train ticket "Zone D" from the airport is also valid for the subway to the "Ruhr-Universität".
How to find Theoretical Chemistry on campus:
From the subway station "Ruhr-Universität" you need to cross the bridge on your right to get to the campus. Pass the library (Universitätsbibliothek) on your right side and walk down the broad stairs until you reach a large square. On the other side of this square is a large round building, the Auditorium Maximum. Go down the stairs to the left of the Auditorium Maximum to reach the natural science buildings. Continue walking on the flat roof until you reach the third tall building, the Chemistry building NC. The office of the Theoretical Chemistry department is located on floor 03 north, room NBCF 03/295, one floor below the entrance you'll take ("Floor 03" essentially means "floor −3"!). Best you use the elevator in the northern area and follow the signs.
For a map of the campus see this GIF (for preview) or this PDF file (for printing).