I think that once you have challenged one another, click on the 'challenges' tab, and your option to accept should be there.
Old Thread: Probs challenging a friend
The problem is neither of us are getting the notification of the challenge so accepting is difficult if you haven't had the alert.
A friend and I are trying to play in Live Chess. I've challenged him and vice versa but neither of us are getting the challenge or alert. Are we being dumb? I've been through my settings and the option isn't blocked. If he challenges me, what is the alert I should get?