Both board meetings will start in the morning at 8:30 (Mon/Tue, respectively), so in practice you will have to fly on the previous evening to "HEL". The EXPReS Board on Tuesday should end before 14:30 so it would be possible to catch flights leaving 16:30 and later at "HEL".
The most convenient way is to take a taxi at the airport. You can get a taxi just outside both the international and domestic arrival halls. There are both regular taxis and the yellow airport taxis. With the latter you can usually share the costs with other passengers going the same way. The taxi operators have service desks in International arrivals hall 2. Taking a regular taxi to Otaniemi costs about 30--35 euros and takes about 20--25 minutes. All taxis take credit cards, at least Visa and MasterCard/EuroCard, and the majority also accepts AMEX and Diners Club. Although the credit card slip will include a line "Extra:___", tipping is not at all customary in Finland, thus you can just confirm the total by writing it on the "Total:___" line. Tipping in restaurants is also quite rare; sometimes when you have received exceptional service you may want to round up the sum a little but that is by no means required and expected.
There is no direct bus connection from airport to Otaniemi. You can, however, take bus 540 or 514 from the airport and change to 52, 510, 512 or 550 in Leppävaara to get to Otaniemi. There is a short walk from the bus stop to the hotel, bus 550 comes closest. The YTV journey planner provides you with timetables and detailed maps. (Enter "Airport" to the "From:" field and "Otaranta 2" (the street address of the hotel) to the "To:" field to get started with the Journey Planner. By varying the date and time you can get descriptions of buses you need to take, complete with maps how to make the connections.)
If you are going to Helsinki city centre from the airport, bus 615 is the easiest option. The standard regional bus fare ("seutulippu" in Finnish) is 3,60 euros which entitles you to connect from 615 to other buses in the Helsinki-Espoo-Vantaa region during 80 minutes since 615 travel start. There is no airport supplement charge in bus 615. If you plan to travel more by bus, check also the other ticket types, especially the tourist tickets. Single tickets can be bought from the driver, and tourist tickets from "R-kioski" newsstands, also in airport.
The Otaniemi campus is easy to get around as everything is within a walking distance. The TKK campus area is just a few hundred metres from the hotel. There is also a grocery shop, a pharmacy and a post office approximately half way between the hotel and "TUAS" building.
To go to Helsinki city centre, you can take a bus from Otaniementie (bus stop Dipoli). Buses 102 and 103 are the fastest,
194 and 195 a bit slower. Avoid buses 505 and 506, as they do not go to
the city centre.