Espoon Keskus is about 30 minutes ride from the centre of Helsinki, whether you come by train, bus or car. You can also use the excellent cycle path network, although it will take you a bit longer to arrive!
Buses 154 and 156 from Kamppi bus station (door 53)
More bus information here: HSL.fi
Local trains E, L, S, U stop at Espoon Keskus as well as intercity trains to/from Turku.
More train information here: VR.fi