Hello! I did a little research on the Lake Bodom and apparently you can get there from Helsinki by train and bus, the bus only goes approximately once an hour (twice an hour during day time on Mon-Fri) so check the time table before you go.

You take the U-train from Helsinki to Espoo station. Bus number 86 leaves right next to the Espoo central station and will take you straight to Lake Bodom. Just ask the driver to tell you when to get off the bus, the bus stop that seems the best is in the crossing of two streets called Bodominkuja and Bodomintie in case you want to check it from Google Earth.

You take the same bus when you want to get back to Espoo center and since the bus goes only once an hour you should really check when the bus goes by again so you don't miss it.

You can check how to get around the capital area of Finland (Helsinki, Vantaa, Espoo) from this site http://www.reittiopas.fi/en/, just write in from where you're travelling and the destination and also time if you want and it'll tell you which way of public transportation is the quickest.

Here http://aikataulut.hsl.fi/linjat/en/e86.html you can find the timetable for the bus line 86 in case you want to print it and take it with you, when it leaves from Espoontori (next to the Espoo railway station) to Bodom and when the bus drives by Lake Bodom so you can get back from there.

I hope this was helpful to you! :)