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:
https://eurohostel.eu/
This hostel is situated at Katajanokka near Helsinki city. From here you can the tram to Arcada

https://www.forenom.com/  
The apartment hotels are situated downtown and close to the tram that takes you to Arcada

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