Helsinki is a popular place to have conferences during summertime. It is good to book the hotels in good time. 

For budget accommodation please book hotel/hostel in these places:
This hostel is situated at Katajanokka near Helsinki city. From here you can the tram to Arcada  
The apartment hotels are situated downtown and close to the tram that takes you to Arcada

Hotels we recommend:
Scandic Hakaniemi
Scandic Paasi
Scandic Kaisaniemi
All three are situated between Arcada (15 min tramride) and downtown (walking distance).