The nonce is the number of the bet you have made with that specific client and server seed. You can see it in the bet information page.
Oh, now I see it. It's in the box with the client seed
http://i.imgur.com/hbAcOdk.pngOk, only the hashed server seed is available. Using the hashed server seed doesn't check out to be provably fair here:
http://i.imgur.com/MwiA8fP.pngSo apparently we really can't check provably fair?
Click your username, then click "change seed pair", and then you can go back to get the unhashed server seed of your previous games.