When you arrive to ELTE-BTK campus enter the garden (the big open space between two buildings, with cars parked on the right side) from Múzeum körút. Here is a map of the campus. Go straight on, till the you reach the end of the road inside the campus (there will be a church-like building on your right and a statue with some bushes and benches in front of you). Turn left and then turn right on the first possible occasion.
This is the D bulding on your right. The entrance of the building is near the end of the garden. When you enter, go down the stairs to the lower ground floor and turn left. The conference room is at the end of the corridor on the right.