There's 2 easy ways this can be done.
1. Click Profile at the top, then go to the "Friends and foes" section thats posted above the registration information bar. Scroll down a bit and you will see pending friend requests and the ability to accept friend requests.
2. Go to the person who is requesting to be your friend's profile and add them as a friend. That will auto accept the friend request since there is a request from them sent to you already.