From the Helsinki Airport you can take the train to the city. It might be easiest just to get a two or three-day ticket as the ferry to Sveaborg is covered by the same ticket.
https://www.hsl.fi/en/travelling/visitors/airport-train
The ferry leaves from the Market Square (Kauppatori) – there are multiple ferries, so take the HSL one. Ferries operate between 06:00 and 02:20.
Schedule: https://suomenlinnanliikenne.fi/en/timetables/
The venue (11 on the map) is located at the red circle with arrow and the main HSL ferry arrives and departs just above it at the pier (1). If you take a ferry that stops at Lonna island, just wait (there is a restaurant on Lonna).
The walk from the pier is 400 meters.
Link to the Sveaborg maps page: https://suomenlinna.fi/en/explore/maps/
