The travel time is about 2.5 hours and the cost for adults 140EEK/9e. From the bus station, bus No 2 travels via the ferry terminal (Reisisadama A-terminal or C-terminal, N.B. DEPENDING ON THE FERRY LINE!), or alternatively, you may travel by taxi. You should allow maybe 3.5 hours to travel from Tartu to the ferry terminal in Tallinn.
On Sunday, June 1st, SuperSeaCat ferries leave at 15:00 and 19:30 and cost 590EEK/38e in the regular class (SeaCat). Catching the latter SuperSeaCat ferry (Terminal A) would require you taking bus #724 (SEBE AS: dep 16:00 Tartu – arr 18:25 Tallinn)
Alternatively, Nordic Jet Line ferries (Terminal C) leave at 17:25, 19:30 and 21:35 (the last one arriving in Helsinki at 23:15), costing 47e for the first two departures and 29e for the last departure. Catching the last Nordic Jet Line ferry would require you taking the bus #158 (SEBE AS: dep 19:30 Tartu – arr 21:55 Tallinn).
Note that on Sundays the ferries are relatively full, so please make your travel arrangements early.
The Nordic Jet Line ferry (Terminal C) that same morning leaves from Tallinn at 8:00 and arrives at Helsinki at 9:40.
Were you to leave from Tartu in the morning, either of these would require you taking the quite early bus #292 (SEBE AS: dep 04:30 Tartu – arr 07:05 Tallinn) and then taking a taxi.
In Helsinki, the ferry arrives at Makasiini Terminal, which is about a mile's walk from the conference venue (see MAP), so you could just make it to the beginning of the workshop at 10:00.