The competition place, airport and Helsinki city center are all connected by the same train line: the Ring Rail Line. P-trains go clockwise, I-trains anti-clockwise.
From the airport the I-train will take you to Myyrmäki station in 15 minutes. From Helsinki center the P-train will take you to Myyrmäki station in 15 minutes. Vantaa Energia Areena is 1km from the station (8 min. walking).
You can also take the following buses from the bus terminal on the south eastern side of the station instead of walking all the way.
571 platform 17, 3rd stop
411 platform 15, 3rd stop
311 platform 17, 3rd stop
Or these buses from a stop close to the bus terminal:
560 Jönsaksentie bus station, 3rd stop
565 Jönsaksentie bus station, 3rd stop
510 Jönsaksentie bus station, 3rd stop