Brussels public transport is maintained by one company, called MIVB. Therefore the same kind of transport ticket is used for metros, trams and busses. You can also ride the train with this ticket as long you stay within the city, e.g. this ticket can be used to travel to the Etterbeek train station from anywhere in the city.
You should get a ticket before getting on. Sometimes a (bit more expensive) ticket can be bought with the tram or bus driver. If you're making a transfer however, you should re-validate your ticket. A ticket stays valid for 1 hour.
Time schedules and travel advice can be found on http://www.mivb.be. We apologize that this site is only available in Dutch and French. Assuming you take the subway to come to the conference site, there is a metro about every 10 minutes. The last metro back to town is at 23:58 (barring changes in schedule).