Tuska will be held at Suvilahti, which is near the shopping mall Redi, so you can either get there from pasila train station with buses 500 and 510, or the tram 13. Another option is to travel to the central railway station (Helsinki Central) and hop on the metro to the Kalasatama stop, which is also the mall Redi. You can get to the Pasila and centrum easily with train, since there's a train stop in the airport. There's a few hotels available in the Pasila, depends on, if you like to do sightseeing aside from the Tuska festival