Jyväskylä can be reached by flight from Helsinki. It can also be reached by traveling to Helsinki and then taking a train from there. Helsinki has a big international airport, international train connections and harbours. The train connections from Helsinki to Jyväskylä are good, with multiple options each day. Tickets can be bought from VR.
If traveling from the airport to Jyväskylä, you need to take either of the available trains from the airport, and change trains at Tikkurila or Pasila. The guide to local trains gives you the best options. Some connections also require changing trains in Tampere.
There are multiple good hotel options in walking distance from the seminar venue.