I think Mirfield's right - you're better off going to Saint-Louis and doing it there.
I seem to remember reading somewhere that the SNCF didn't really want to run that part of the station any more and were hoping to give it over to the SBB - which is why the only thing left is the one solitary ticket machine.
The last three times I've been there I've seen people miss their trains because of massive queues behind that stupid machine. I even ended up buying tickets for half the queue once because I was the only one there who could work it