Hello and welcome to the game!
Poromagia in Pasila (Opastinsilta 7 B) is indeed the magic central of Helsinki. You are likely to find players and traders there on most days, except maybe tuesday when the shop is taken over by figuregames such as warhammer.
Wednesdays there is always a modern tournament there, and if you hang around before it starts at 17.00, someone is likely willing to playtest with you.