You should get a train to Coimbra-B railway station.
Depending on the train, it can take you about 1 hour to get to Coimbra. You can check the timetable and buy the ticket online from CP. When buying tickets search for the AP or IC services, as these are the better trains to catch. Prices can vary from 19.50€ to 33.30€.
You can take the taxi to Convento São Francisco which is about 10 minutes ride with an 8€ fare but a better choice is to come walking to the conference hall, contemplate the magnificent view of the town with a 40 minutes riverside walk.