Unable to sink first gen. wireless keyboard.

I have an iMac 24" EMC 2111 intel core 2 duo running OX Lion and have been trying to sink a first gen. wireless keyboard with the with the Mac. It seems to find it, but when its time to type the pass code to link it, the keys will not function to type. The Apple site says I need to refer to the original instructions which I don't have, although it does say a USB keyboard and mouse is needed to set up this model. How is this done? The batteries are fine and the LED blinks like others I've done. Its a KY432OODGPMHA. It has worked before on a 20" Mac from the same period. Thanks.