Hi Emmanuelle,
The Arenal Volcano is about 4 hours far from Cartago (Where the OpenACS and .LRN Conference takes place). According to the plans we are going to stay there from 5 at nigth thru 6 at after lunch then we come back to Cartago. So around 6 pm on 6th Friday November the bus will across the center of the country (will be at the airport Juan SantamarĂa, Alajuela) so I suggest you to reserve in a Hotel around the airport and plan your flight on Saturday or Friday night.
I will specify more the tour on the web site.
If you have any questions be free of asking, I'm glad of help.