I had the exact same problem yesterday after completing the campaign. Actually, the arena logo won't always show up on your map for some reason, or until you find it or something... Anyway, just go to Shanath (you can teleport there. It's the arena place where you did stuff for Noore), climb up the stairs and you should see it!
Have fun! I hope it works for you but it did for me, so good luck!![]()