I think dueling is broken right now - but if it does work, you usually just go to 'My Stuff', then 'Duels', then you should see the challenge.
It'll take you to a page that has these links at the top:
|Challenge Someone| Open Duels| History|
Click 'Open Duels' and you should see the challenge, which has a link you can click to accept.
Hopefully, it'll take you to a screen where you can pick which gesture to throw out. More likely, however, it'll do this to you. :(