When you go into the arena you are given a list of people. At the bottom of the screen type in the name of who you think you might want to fight. Click go. Before fighting though click on the person (or their name, I can’t remember) it should show their stall.

At least I think so, I can’t remember. I usually register for the tournament and click on people I fought and then to their cell and who they had fought to their cell.