What is the best way for four people to get to the Brussels Conrad Hilton on Ave Louise from the airport?

We have a package booked this month but are doing our own air. I was thinking that we could take the train from the airport to Gare du Sud and then the Metro but don't know what the cost would be or if there is a better way for a small group. I know that the hotel is a few streets away from a Metro stop. Thank you for your help.