IMac Desktop setup issues.

My mother was given a "used" Mac i5 Desktop computer that I am trying to help her set up. The keyboard is the older wireless style that needs 2AA batteries. During the initial setup of the Mac I am asked to type in her wifi password and user name etc... the problem is that the keyboard will not type. I do not think it is communicating with the computer - it does have power to it (the lights light up). What can I do to get it to work so that I can finish the initial setup of the computer. I know that I can pair a new keyboard later in system settings... but I don't know what to do at this setup stage. Please help!!!