The Helsinki Institute of Physics is located at the Kumpula science campus, which is located on a hill four kilometres north of city centre of Helsinki. A good landmark that can be seen from afar is the weather radar located on the roof of Physicum.
Kumpula campus, location, instructions and parking.
Public transportation timetables can be viewed from the HSL journey planner
Arriving from the airport, you may take the bus 615 or 620, or the P-train. These have stops within two kilometre distance from the campus, and there are a number of connections that will take you directly to the campus (see the journey planner link above for details). The local transportation ticket can be bought from the kiosk at the airport or directly from the bus/train. Ticket prices can be seen here — note that you need the regional ticket to travel from the airport to Helsinki.