Train in Brussels

I have a brief layover with a team of nine college students in Brussels in a couple of weeks. I've had a layover in Brussels before, but I had trouble last time with the train ticket from the airport to Central Station. For starters, I arrived so early in the morning there was no one working at the ticketing counter. Then, the machine I should have used to obtain tickets was out of order. While I was under the impression I could get into Brussels for a few Euro, the only working ticketing machine charged me over 7 Euros. Upon returning to the airport, I learned the ticket I had was not valid and was charged an additional 10 Euros. How can I easily obtain tickets and avoid headaches with my team? Can I use a credit card to obtain tickets or do I need to get Euros in advance? Thanks! Brant